for the v-wheels: put the excentric nut all the way open, roll the carriage and feel how smooth it is. If you yank it up and down/backand forth you'll feel some play.
Now incrementally tighten the excentric nuts, you need to find the sweet spot where it stays that smooth while reducing the play to zero.
for the belts, my experience is that you can tighten them as hard as you MANUALLY can tighten them. if you use tools of any sort, you'll probably end up with too tight belts