Short of being corroded into place (which I have never seen but it is possible), or the size of the bar was incorrect as discussed earlier, there is no reason someone would have to pry the stem apart to get it to release the bar. In fact if the right size bar is used the bar should slide freely inside the clamping area of the bar even at the bends (though at the bends you do need to turn the bars a bit to get it past the bend.

Prying on a aluminum clamp that wasn't designed to be bent could result in a cracked stem clamp, so extreme caution needs to be taken when attempting such a move. If the stem is going to be replaced then no big deal if the old one cracks. The best way to open the clamp if you need a bit of space instead of using a screwdriver is to unscrew the clamp bolt and insert a penny into the slot, then tighten the bolt against the penny, this will open the clamp a bit, but don't open more than what it will take to make the bar slide. Leave the clamped penny installed while removing the bar. You can do the same thing when reinstalling either with a new stem or a new bar.

You could also a quarter, BUT, if the aluminum clamp is of the thick variety it could crack it, so do so but only at your risk. It shouldn't have to open more than 0.5mm, any more than that it you risk damaging the clamp.

There are some designed stems that the penny trick will not work, so it depends on the stem you have.

Also you can use hand soap to help the bar to slide easier once it's loose.

Even with that bit of space scratching the bar will occur, however only around the bends of the bar which tape will cover anyways.