We went to Disney a few years ago in early May and there was one of those cheer competitions going one. Had no idea until we arrived and never noticed any issues. Typically, they stay at one of the budget Disney hotels and during the day aren't in the parks. We assumed they were either practicing or busy with one of their various competitions. Only if you are staying at the same hotel might you notice a difference. Those events would make no difference to us in planning a future Disney trip and have zero impact on deciding which parks to do on a specific day.
As I recall the discussions on here, the marathons tend to block off certain roads for some period of time that are part of the competition. I would assume Disney provides routes to use to get around the blocked off streets. Usually there are several threads on here talking about the various marathons.
