machine work questions.

Yes by all means! The plates will pull the block/ bores more into line with where they'll be when the heads are torqued down. Ask some of the guys on here that are actual machinists (MRL, Moper, OU812- Brian) & they can probably tell you how much a bore will move when the plates or heads are torqued down. It's significant enough.
You can do it without plates, like if it were just an ordinary rebuild & be ok, but you'll be leaving some power behind if you don't.