My fronts & sides are towers, my rears are also KEF eggs. I think it sounds great. My wife agrees. I'm replacing my front towers, and will move them to the rear. So side and rear surrounds will all be nice towers. Skeptical how much difference it will make, but some people here have exceptional speakers all the way around, and say it does make a big difference.
I would say do the best you can with the Front L/R and Center. That is where most of the sound comes from in TV/Movies, and the only place normal stereo music will be played. Spend less on the sides, and rears if you have 7.1 or 7.2. Over the years, if/when you upgrade your fronts, you can move them back to surround duty.