When someone rerolls a champ, they should go into a "pool" for your team to take them from. That way when someone forgets to ask or is to hasty about rerolling a champ someone else wants, they can still take it.
Just a thought.
A good rule of thumb most players follow in ARAM is simply this...
Reroll only after trading has concluded or declined unless there aren't any trades available.
It is an unspoken courtesy that is generally extended between players because of the extremely casual nature of the mode.
