I have read a lot of articles about how you should never burn softwoods because they will cause creosote to build up in your chimney, and if you do your house will likely burn down. The idea is that the resins in softwoods cause the creosote to build up. But these are mostly rumors.
Creosote buildup is primarily caused by the chimney flue cooling the exhaust gasses enough to where they start to condense onto the walls of the flue. This is the case with both hardwoods and softwoods. If these exhaust gasses are above 250 degrees Fahrenheit, they will rise out of the chimney the way they are supposed to. When the temperature of these gasses falls below 250 degrees, they condense on the flue surface and build up.
There are a couple main causes of low flue temperature. The first and maybe the most common is the wood burning appliance operator. Burning wet wood releases water from the wood and absorbs heat from the combustion and the exhaust gasses, causing it to cool. Burning wet wood is a big cause of creosote buildup.
Restricting the oxygen supply or dampering down causes the wood to burn more slowly or even smolder. This may be good for holding a fire all night, but it creates a lot of particulates and unburned gasses without a lot of heat. The cooler and dirtier gasses will likely cool enough as it rises up the chimney to deposit creosote.
The best ways to prevent this is to burn dry wood and burn hot fires instead of smoldering fires. If you want less heat, instead of throttling the air supply, simply put in less wood and make a smaller fire. This is easier done with smaller pieces of wood.
Now back to softwoods causing creosote. Resinous softwoods are well known for burning quickly but more intense. A lot of people don’t like softwood because a lot of the heat goes up the chimney instead of radiating into the room. With more heat going up the chimney, there may actually be less creosote.
The other main cause is poor flue design. If you are burning hot fires with dry wood and still having problems with creosote, you should have an expert come inspect your system. Some poorly designed flue systems can cause the gasses to cool too fast no matter how you burn wood below.
On the flip side, there are some chimney flues that are so good that you can burn all the wet wood you want without a problem. This is partly why there are so many conflicting stories on this subject.
Even in the best cases, all wood burning flues will have some buildup and all flues should be cleaned regularly. This article is only my opinion. Contact a trained professional for real advice. If you do a search, there are a lot certified chimney cleaners who have websites with great information on keeping your flue safe and clean.