I just put some Roto 77's (105-45) on my Geddy and the neck bowed like crazy. Can I use the heavier tension flats on this bass if I adjust the truss rod, or do I have to switch to lighter tension ones?
Either go lighter tension or adjust that truss.
I tried a set of those same Rotos on my 59 CS P-Bass and changed them out the next day for a set of D'Addario Chromes. Not only did the Rotosound strings torque my neck, they were way too bright. The D'Addarios have a nice deep warm tone, and a lot less tension (just the right amount IMO). And the neck settled back down too once I strung them so I didn't have to mess with the truss setting.