However, there IS a ceiling. If you're rolling by yourself in a tier 5, you will almost never be put into a match with tier 10's. It's usually around a 3 tier difference between the lowest tank in the list and the highest tank in the list.
IIRC, tier 1 & 2 tanks won't go into matches with anything higher than tier 3. Once you hit tier 3, there's a hard "cap" of 4 tiers. Though there do seem to be exceptions for the top light/scout tanks (Leo, M5, A20).