On my -4, I put the (early model) GPS under the panel. Hated the location, but I was out of room.
Given the choice, I'd put as much as I could on the main panel so you don't have to bend down to see it. And it's much easier to work on things there, too, after you take off the fuselage panel.
RV-9A at KSAV (Savannah, GA; dual G3X Touch with autopilot, GTN650, GTX330ES, GDL52 ADSB-In)
Previously RV-4, RV-8, RV-8A, AirCam, Cessna 175
ATP CFII PhD, so I have no excuses when I screw up