Creosote can ignite easily at 450 F degrees and it takes only a little bit of creosote to cause a chimney fire so it's always necessary to keep your chimney cleaned of creosote to prevent a chimney fire.
It takes as little as 1/8" of creosote to ignite and cause a fire so clean the chimney yearly just to be safe especially if you use the fireplace or wood stove often.
To prevent dangerous creosote buildup always make sure you're burning dry seasoned wood that has been cut, split and stacked for a minimum of 6 months to a year.
Letting firewood season a year is much better to reduce the moisture content to allow an efficient burn and prevent creosote buildup.