This a tougher question than it sounds. The quick simple answer is: The National Fire Protection Association standard 211 says:
"Chimneys, fireplaces, and vents shall be inspected at least once a year for soundness, freedom from deposits, and correct clearances. Cleaning, maintenance, and repairs shall be done if necessary."
This is the national safety standard and is the correct way to approach the problem. It takes into account the fact that even if you don't use your chimney much, animals may build nests in the flue or there may be other types of deterioration that could make the chimney unsafe to use. The Chimney Safety Institute of America recommends that open masonry fireplaces should be cleaned at 1/4" of sooty buildup, and sooner if there is any glaze present in the system. Factory-built fireplaces should be cleaned when any appreciable buildup occurs. This is considered to be enough fuel buildup to cause a chimney fire capable of damaging the chimney or spreading to the home.

3G Chimney does not recommend “cleaning logs” as a substitute for traditional chimney cleaning. The best explanation of the matter could be found on CSIA web pages.

There are a multitude of reasons for smoky fireplaces and stoves. Some reasons include: A build up of creosote or an animal nest resulting in reduced draft.
What to do? Clean the chimney.
The damper may be closed or partially closed.
What to do? Look up to the damper area and see if you can see it open you may have to be moving the handle while you do this to see it ... watch your eyes for falling debris.
The Flue is too small for the fireplace that it is servicing.
What to do? A simple test this by placing a 4" to 6" wide strip of aluminum foil across the entire top of the opening of the fireplace (where the smoke would be rolling out from). Burn a fire and see if the smoke is stopped or reduced. If it works a piece of metal can be made or purchased to stay there permanently. This is called a Smoke Guard. The more elaborate and expensive answer is a draft induction fan placed on top of the chimney to make it draft.
Chimney is too short.

The smell is due to creosote deposits in the chimney, a natural byproduct of wood burning. The odor is usually worse in the summer when the humidity is high and the air conditioner is turned on.
A good cleaning will help but usually won't solve the problem completely. There are commercial chimney deodorants that work pretty well, and many people have good results with baking soda or even kitty litter set in the fireplace.
The real problem is the air being drawn down the chimney, a symptom of overall pressure problems in the house. Some make-up air should be introduced somewhere else in the house. A tight sealing, top mounted damper will also reduce this air flow coming down the chimney. The chimney may also be leaking water into the masonry around the top of the chimney.

This has become quite a common problem in modern air tight houses where weather proofing has sealed up the usual air infiltration routes. The fireplace in use exhausts household air until a negative pressure situation exists. If the house is fairly tight, the simplest route for makeup air to enter the structure is often the unused fireplace chimney. As air is drawn down this unused flue, it picks up smoke that is exiting nearby from the fireplace in use and delivers the smoke to the living area.
The best solution is to provide makeup air to the house so the negative pressure problem no longer exists, thus eliminating not only the smoke problem, but also the potential for carbon monoxide to be drawn back down the furnace chimney. A secondary solution is to install a top mount damper on the fireplace that is used the least.

Without a doubt! Although gas is generally a clean burning fuel, the chimney can become non-functional from bird nests or other debris blocking the flue. Modern furnaces can also cause many problems with the average flues intended to vent the older generation of furnaces.

In case of a chimney fire, follow these steps:

1. Call the fire department immediately.
2. Alert others in the house to evacuate.
3. Close the appliance’s dampers and/or the primary air inlet controls, limiting the fire’s air supply and reducing its intensity. If there is a barometric damper in the chimney connector, plug or close the opening in the barometric damper.
4. Open the appliance door just enough to insert the nozzle of a 10 lb. dry chemical fire extinguisher rated for Class ABC fires. Discharge the entire content of the extinguisher into the appliance and shut the door.
5. If possible, wet down the roof and other outside combustibles to prevent fires ignited by shooting sparks and flames.
6. Closely monitor all combustible surfaces near the chimney. During severe chimney fires, these surfaces can become hot enough to ignite.

After a chimney fire, have the chimney inspected by a professional chimney sweep or woodstove/fireplace installer. Contact your insurance carrier. Do not use the chimney until a professional has inspected it.
The excessive heat produced by a chimney fire can crack chimney walls, damage chimney liners, and damage some types of factory-built chimneys. If not repaired, these damages create a greater possibility for any subsequent chimney fire to spread beyond the confines.