When it comes to the amount of fur Dominique, a.k.a. Dom the Pom, has at different times of the year, she's kinda like a chameleon - shorthaired in the hot Florida summer and very poofy (her nickname is Poofer) in the winter, even though our winters are mild.
A while back I read something about a longhaired dog's fur being a form of insulation, keeping the dog cool in the summer and warm in the winter. I never have her trimmed, partly because I love her Pomeranian fluff.