Is it better practice to traverse a portal of related records to change data within, or just to go to the related layout, find records, and traverse and change those records from beginning to end?

IMHO, a portal is UI element, part of the presentation layer. If you want to work on the data layer, I don't see why you should bother with it. I haven't tested the actual performance, but from the point-of-view of writing the script (I assume that's what we're talking about) going to the child table is much more straightforward.

BTW, the GTRR step has an option to create the found set directly, so no find is required.