Better speakers = better sound. You just won't unlock their full potential. It'll still be a noticeable improvement. I'm a fan of having most of my sound coming from the front- like it would at a concert for example. I'd suggest rather than spending money upgrading the front and rears, buy better fronts only- which will then most likely result in better ranges (highs and lows), and the result will sound much cleaner.
You can always add later! Start off the right way! Find a real audio shop as well to talk with them, hear the speakers. A good one that will spend time with you discussing the options, not just trying to make a sale.