Filling out disks - please advice

Is it best to make dummy-files to expand the games to exactly 74/80mins?

Does it make a big difference when burning, for example a 684MB large game on a 80min-dics, to make a dummy-file for it first or not?

And is it failsafe? Is it often that games won't work after adding a dummy-file?

Please advice
-=-BagronkeN