This is getting rather distant from the original Q
but I find that entering the entire flight planned route is very useful.
The IFR sector controllers do have your filed route in their system and they will generally give you DCTs to waypoints that lie
on that route.
With any GPS, it's a lot quicker to get the cursor up and select a waypoint already there and press DCT, than to enter one from scratch. It's also a lot easier to identify an unfamiliar intersection/navaid name from a list than to ask ATC to spell everything.
Personally I find that - apart from general weather avoidance tactics - locating unfamiliar intersections that ATC throw at you is the major workload in IFR flying.
I would not say that "RNAV" automatically means GPS. Bendix-King make both the KNS80 and various IFR GPS receivers. It indeed should mean GPS these days, but there is a lot of old planes around with a KNS80, which is practically useless.