the screws are M4 threaded. If you are worried about them getting seized, loosen them when they are new and apply a little copper-ease or something.
They are important. Campag don't use screws but I am told that comparatively recently they have revised their brake block holders to include a spring clip which does the same job, i.e. it prevents the brake block from falling out should it become loose for whatever reason.
One reason the screws are important is that the fit of the brake block inserts can easily deceive you. One reason for this is that rubber and plastic mouldings can continue to change size and shape after they are out of the mould; often this process carries on for months or years.
For example I have struggled to fit new inserts into new holders, (to the extent that I couldn't have easily removed them) only to find that they would practically fall out by themselves six months later. I wondered if being on the bike had caused this somehow; forunately I had assembled a spare set, and those had sat in a drawer unused. They were just the same.
Even if they are tight to start with, there is basically no way to be 100% sure that brake block inserts that you buy won't shrink in this way, so if you are tempted not to use the screws, think again.
cheers