I didn't do a theme my first few years of teaching because I was too busy digging into the curriculum. Now that I'm more comfortable with that, I find that decorating my room a certain way is really fun! I don't spend a lot of money on it, but I like how it seems more uniform. If I have it decorated at the beginning of the year (before the kids start) then I don't have to think about different bulletin boards, wall hangings, etc... during the year.
The kids really love it, so that makes me even more pleased to do it.
I don't think it's "necessary", and if you don't feel like it then I wouldn't do it. There's no need to make added stress for yourself.
