With a trenitalia 5 trip pass , do I need advance reservations?

You can't get on Trenitalia's high speed trains without a seat reservation, but you can get a seat reservation - just like tickets - up to 15 minutes before the scheduled departure time.

Depends on what you mean by "advance reservations." With all train except Regionale, you must have a seat reservation prior to getting on the train even with a pass.