It is generally a good idea to keep the teams balanced. However, I also believe that switching people ‘just because one team is winning’ is a pretty lame excuse for switching players around.
I personally only balance when one team doesn’t have enough players OR there is a clear clan stack going on.
The idea that a team has won 5 rounds and lost 0 rounds can’t, in and of itself, justify a team balance. There needs to be something ‘wrong’ that the players on the winning team are doing (namely clan stacking) in order to really have a good reason for switching people. In many cases I find that just switching 1 or 2 people can ruin the winning teams formula and simply flip flop which side ‘seems’ overpowered 
.
That being said, I will on occasion switch people that I know are excellent players who could help out the losing team.
Again, it is a subjective argument. Nobody likes being on the losing team. If the circumstances seem appropriate, I will balance the teams. Ultimately, it is each individual admins choice.