I've rebuilt many racks because I didn't initially give myself enough space. I now go no higher than 2 tanks high, even with small tanks. Going in and out with nets, hoses, filters, and everything else calls for at least 12 inches clearance above the top of my tanks. Everyone is different, but the more space you leave yourself the less you will want to rebuild them later.
Specializing in breeding Philippine Blue and Wild-Cross Angelfish