Need a little clarification. In some posts I have read that one should open the brake bleeder screw when pushing the pistons back or you risk damaging the master cylinder.

However, most of the posts stated that (including the DYI link above) do not mention this. Does one need to open the bleeder screw when pushing the pistons back.

When I replaced the pads on my 91 300E I never opened the bleeder screw.

No damage I know of in any vehicle, except the risk of not subsequently eliminating all contamination. The most deterioration and contamination of the brake fluid will occur at the caliper cylinders, and pushing them in far enough to install new pads can move the grungy stuff all the way to the master cylinder. Since you should be bleeding the brakes anyway, draining the caliper cylinders is a prudent move.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

I've heard this is a valid practice for good brake system hygiene, for the reasons Steve mentioned. It's one of those steps that not everybody does, myself included. Then again, I flush the brake fluid everytime I do the brakes.

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95 E320 Cabriolet, 159K

I have posted the procedure a few times over the years and the
reason I have is because most just do not change their fluid
[ why, is beyond me] ..
So, this is just a second best procedure that makes for both a better/cleaner job, and an easier way to push the caliper back in.
At least you get the main crap out and get to see the fluid condition...
Why it is never mentioned in DIY atricles has always been a mystery to me , as most brake guys all do this without even thinking about it.
Why NOT doing it should be the question.....

I think some are just fearfull that once they even get near a bleeder, they are in for the" AiR in the Sytem Nightmare "...

Putting the bleeder hose in a container with fresh fluid before bleeding will eliminate any chance of getting air in the system.