Stock racing is all about battery power..the higher amp rate you charge, the more power you will have, but the shorter the life of the battery will be. Charging at 4 is easy on the packs, and peaking at 6 will "shock" the pack into having punch. At big races, I could care less about battery life..I want power! So I will beat my packs up and charge at 7.
In mod, punch isn't as big of a deal..if you need more power, you just drop in a faster motor (my cars wheelie almost uncontrollably..the last thing I need is more power!). My dad likes to charge and peak at 5 amps, but 4 amps is perfectly ok.
I don't change my charging parameters according to anything other than the importance of the event. There are many other ways to adapt to track conditions.