I don’t like to make extra grooves in my MFT. When the work is small enough to fit there but too long for the fixed rail (or I just don’t want to reposition the fence for just a couple cuts) I slap a loose piece of 1/8” tempered board on top as the sacrificial backer, and make the cut with a loose rail.
I also use one or two as shims when cutting thin stuff and don’t want to lower the fixed rail.