A properly installed toilet generally does not leak! The key to keeping a toilet from leaking, other than some catastrophic failure where something cracks or breaks, is to ensure that the toilet does not rock or move once set. The most common material to set a toilet is wax. Wax is not like a spring. If the toilet can move, it can break the seal. NOrmally, the horn of the toilet is much smaller than the opening in the toilet, so even if that seal wasn't good, it would still funnel it down the drain, and nothing happens except sewer gasses can escape. But, if there's a backup, and the seal is compromised, yes, it can and unusually will, leak.
So, you only get one chance to set the toilet when using wax. You need to check and accommodate any shims PRIOR to setting the toilet down the last time into the wax. Once it is set into the wax, snug up the bolts. Then, to help prevent it from moving, you caulk around the front 3/4'ers of the toilet. This locks in in place. Using the proper caulk means you can still pull it up later if required (i.e., not silicon!), but leaving the back open, tends to show if it leaks.
Ideally, the toilet flange is installed on TOP of the FINISHED floor, and tight to it with no gaps. If the flange is lower than that, you MIGHT need either a jumbo, or to stack wax rings to ensure a good seal. There's a 'sticky' at the top of this forum that describes how to properly set a toilet. IF you follow that, you should not need to worry about leaking.
If your flange is hanging in thin air, not screwed down to solid material, you need to do something about that to make it solid.
