I recently learned that you have to be careful with those drop-in-the-tank cleaners. Make sure they don't contain bleach. Those will erode the flapper.
Alternatively, you can get something like this:
$1 It's a box with one tube coming from the flush valve and another tube aimed down the refill tube. The box holds replaceable cleaning cartridges. The cleaning chemicals flow directly into the toilet without touching the flapper valve or anything else in the tank, so you don't have to worry about erosion. And since the water only flows when you flush, it only activates the cleaner when you flush, so it's not constantly being dissolved by the tank water.
I'm not a sales rep or anything. I just found out about this thing the other day, and it sounds kinda cool.