As an aside, you may really wish to leave them on.
* TiVo never records a suggestion when you have told it to record a program in a given timeslot.
* TiVo never deletes programming you have told it to record to make room to record a suggestion.
* TiVo does not have a free space indicator -- but because of the first two points, if you leave your suggestions on, you can get a rough idea of how much space is left by looking at the number of suggestions recorded. The higher the number of suggestions, the more space is available.