If you got it direct from Nokia or a store, reflashing isn't important. If you got it second hand, you may be better off just reflashing anyway, to start from a good/known point. The previous owner may have installed and uninstalled things and left odd settings.

The first 10 to 15 minutes after a flash install or initial turn-on, it may be a little laggy, since it's doing one-time setup things (tracker, etc). It may also be doing lots of indexing if you put a microSD card in too, scanning it for media, etc.

You may want to look at some of the "best of" or "must have" app threads here to get an idea of some of the tweaks and tuning you can do. There's quite a lot to tweak, and doing that out of the gate is usually best. (Since if you screw it up and need to reflash, you're not losing months or years of data/config, etc.)

Welcome to the N900!