(Below, I'm revising my previous post. See additional comments in italics - they revise my original text in the quotes.)
I've used the pass. Probably will use one this May again.
*You do not need reservation fees for any of the eligible trains. (If
you WANT seat reservations you can buy them at the standard price.)
This is true for 99.9% of German trains but NOT true for the eligible EC train from Munich to Venice - you need a seat reservation.
https://www.bahn.com/en/view/offers/other/seat-reservation.shtml
It's not like the French rail system - DB does NOT allocate a limited number of seats for rail pass holders - you can just get on the train - as long as it's one of the "DB-OeBB EuroCity trains" - when you travel Munich-Venice. (You cannot use the rail pass to board just any train or sequence of trains.)
*So once you have found an eligible train, there's nothing to do but
climb aboard (and enter your travel date on your GRP of course.)
But climb aboard with your reservation in hand. If you don't have one, I believe the train personnel will compel you to buy one from them. Not sure if they are priced the same in this situation.
The EC 87 from Munich to Venice is direct and eligible - it leaves at 11:34 on the date I checked.
GRP flyer link