Phil, you're going to Thailand in the "mild" season when rainfall is rare, so the risk from DF is lower than in the rainy season. However, don't let your guard down, precautions against DF are always necessary. The pesky mozzie that carries DF is active all year round. It just needs a pool of stagnant water to breed, that can be inside an old car tyre, in a pot plant or an old tin can. If you see stagnant water around your accommodation, tip it out.
2010 was a particularly bad year fro DF in Thailand, with about 3 times the number of cases from previous years, so 3 times the risk of you getting it.
A work mate recently caught it in Bali, and he says he wouldn't wish it on his worst enemy - very painful.
Thailand in February: LOW risk, but not NO risk of DF. almost 8 years ago
I would definitely use precautions against Dengue Fever while in Northern Thailand any time of the year. As Phil Sylvester stated, your chances are considerably lower during the "mild" season...but don't let that take away all your concerns! I was traveling through that part of the country in the end of July, 2010 and ended up catching Dengue Fever in Chiang Rai. NOT FUN. A couple of weeks in bed...in and out of hospitals. Please don't let that be you! Better safe than sorry... Safe travels! :-) almost 8 years ago