I always stick the toilet down with silicone whenever an underfloor heating mat has been installed. I know the mat shouldn't actually be under the toilet, but am too scared to even think about drilling anywhere near it.
Once stuck, as matthewm1965 says, they don't move.
I actually had this dilemma a few weeks a go. In the end, after thinking long and hard I went with screwing to the floor (tiled) with the screws/bolts that came with it, and nice chrome screw caps. If in the future I need to get to water inlet/soil pipe (everythng is enclosed/back to wall toilet) I just undo the side screws and slide the toilet pan out nice and easy!
I've been in a position in the past were pans have been stuck down with sealant and its a nightmare prising it from the sealent, then you've got all the sealant to scrape away if you then re-seal.