When my IS3 gets destroyed down to zero % it costs about 12,600 credits to fully repair back to 100%. Add in however many shells you need to replace, at 1,026 credits each, and you can see how expensive an IS3 is to fix up and resupply after a fight. I been told WoT devs did this on purpose so it would be hard to play the higher tier tanks. Something about forcing people to not play the top tier tanks only supposedly. Russian logic, go figure. Anyway, so, yes, you will want to keep some tier 5-6 tanks in your garage to help generate revenue to support your heavy tanks.
Yesterday I bought two new tank slots and bought myself a pair Tier 2 tanks. The T26 and the PzII. I missed fighting at this level with those tanks. 80% of the fights had no arty in them. can you believe that? Just straight out tank vs tank fighting! Everyone rushing around and mixing it up amongst the enemy. Ah what fun it is! Quite a few experienced people playing alongside the newbies too. Don't get me wrong. I like fighting in my middle tier and upper tier tanks but sometimes its just fun to go back to the training levels and just having some simple fun.