I will second Bob Leffler's suggestion for access in particular through the baggage floor. I ran one 3/4" plastic conduit under the rear seats and called it quits for "access" there. I have the two 3/4" plastic grommets on each side installed that handled my battery/static tube/AP/Txpder/trim/misc wires to the back, I did find the extra tube handy for the GPS antenna RG400 wire (Garmin GTN650). However, if you add/install an inspection plate on the left and right sides above and a little forward of the step (baggage compartment), you really have access to under the seats as well, especially easily if you have some conduit or just holes with grommets installed BEFORE riveting on the floors. I also very much like the idea of access to the step mounting bolt etc...... Just as a FYI, I made the panel on the right-side of the baggage compartment removable (add nut plates on flooring & side angle) and this provides access from front to the rear, should I ever need that one last wire run to the back that I forgot......
Another comment, since you are really planning how to get wires to the back with access, I used the right side 3/4" angle/ longeron as a SECONDARY ground to the front (engine/firewall), some have run large copper wire, some nothing at all. I can & will just say, my single PC928 in the back cranks my heavy Hartzell prop beautifully on the hottest day of a hot start. And at KIYK these days at 110+ deg F, I understand a hot start for the engine and me in the cockpit!
Dues paid 2017
RV7 built & sold, 500h of pure joy in the air: sold
RV10 slow build & flying & loving it