It does help to have matched speakers across the front three channels as they represent one continous soundstage. If you use the same companies speakers and drivers you're less likely to get noticable timbre differences between the speakers. However, placement of the speakers can also change their sound, so be careful. Ideally, you should try to get all the tweeters at about the same height or at least pointed at the same place.

If you match your surrounds, you may get better front to back pans for the same reason as above. It isn't as noticable to the overall sound if they're not matched with the front soundstage.