Are they small enough to just be single pixels? You can download some free little tools to check for dead or stuck pixels - basically just little scripts that sequentially turn your screen all black, then all white, then all red, then blue, then green (I have not used any such app in a long time, but I'm sure they are out there at macupdate or another Mac freeware/shareware apps site).
If everything else about the machine is fine, it could well be a hardware glitch (although not necessarily anything really terrible, just annoying perhaps). If so, and you are under warranty, be sure to take advantage and get it checked out. If not under warranty, and it is not getting worse and is only during boot up, then personally, I would probably not worry about it until or unless something got worse.