If you have two teams in the same League or Division and they are playing each other then it's really up to you. You can field a weakened team against the one that is more important to get the win, it isn't considered cheating it's simply a perk of having two teams.

I can't police manager team selections, all I would say is would you really want to do that? I mean in the spirit of the game and to be fair to other managers around you? It's really your choice and what you feel comfortable with. I generally find that managers still put out their strongest teams so it hasn't been an issue so far.