Sometime in the distant past, somebody thought it was a good idea to crack open the bleeder while pushing the pistons back in even though THERE IS NO MANUAL THAT EVER SAID TO DO THIS except possibly for the most recent vehicles.
The only way I'd even consider doing this is if I'd already installed 1-way bleeders (aka: 'speed bleeders'). Doing this on conventional bleeders (and just leaving them open while you push the piston(s) back in) is going to introduce air into the system, which will have to be bled out.
If you don't do it before putting the vehicle in service, it's going to be more trouble, later (because the air doesn't just stay at the end of the line). At that point, you may as well replace all the fluid, anyway.
Knowing the way lazy Americans take care of their cars (which means, 'no one on this board'... lol), as long as the vehicle's brakes operate when pushed, they're not adding fluid as the pads wear. They're lucky to keep their oil changed. So there's probably plenty of room in the reservoir, on average, to handle the (now extra) fluid displaced by reseating the piston. The exception is when you purchase the vehicle used; then you don't know what's been done (and most of us would prolly replace all the fluid at that point, just on principle.)
Now... that said... would I loosen up the cap on the reservoir, while pushing the piston back in the seat? Yeah, I think that's good practice. I try and do that, myself. That also allows me to note how high the fluid is in the reservoir, too.
Finally, remember that the 'full' mark on the reservoir isn't at the tippy-top - it's at a lower point, where there's still room for fluid. So under those 'normal' circumstances (no one adding fluid as the old pads wore down), it *shouldn't* overflow the reservoir. Especially from doing one caliper at a time, and then checking the level.
I keep a $1 store turkey baster in the garage for removing any excess (and also for removing as much old fluid as I can, before starting a fluid replacement.) No sense in sending it out bit by bit through the caliper, when I can easily suck it out at the other end & replace with clean. Shortens the time to get the remaining old stuff out of the lines.