I've renovated a few houses, and whenever I replace a toilet the first thing I do is replace the shut-off valve.
Unless they are frequently used, the older style shut-offs with the round handles tend to fail after sitting in the same position for years. Assuming you can still shut them off, they'll either start dripping when they are shut off, or start dripping when you turn them back on again.
The newer style shut-off valves are simpler and much more reliable, requiring only a quarter turn between on and off.
Replacing the valves is easy and inexpensive, except when it isn't.
I once tried to replace an old, very green and corroded valve in a dark corner. The supply pipe was too long anyway, so rather than attempting to de-solder the connection, I simply cut it off with a pipe cutter.
The cutting took longer than normal, and once I had it off I discovered why. The supply pipe wasn't standard copper, but brass, with threaded connections, and I had just destroyed it.
There was no way I could un-cut the pipe, and since it was thicker than copper pipe I couldn't attach any shutoff valve to it. To make things perfect, there were two families in the house that now had no water at all: time to call a real plumber and fork out the $$$.