The approach I've taken with backs for a long time has been mostly opportunistic. It was far easier when I started 30+ years ago, but works well for me considering the two things I lack - concentration and budget.
So when I used to look through a stack, I'd look at both fronts and backs. There were some tough choices and a few easier ones. Like do I get a couple fairly nice commons, or slightly more but with "tougher" backs but not as nice.
My goal had been to get one back from each company, and I got close. Just missing Drum, Uzit, and Ty Cobb. And Maybe AB I don't recall for sure on that one. (Lets not go "there", it was on the list so it was on my list
)
Along the way I picked up most of the factories and series too, missing the ones you'd expect plus for some reason a SC150-649. I had a bit of a preference for the 460 series and minor leaguers.
I'd think a basic set of backs could be put together easily with just a few more holes without it being a major budget issue. Figure you'll spend a bunch on Broadleaf , Lenox, and CB. The real tough ones will take some budget, some luck, and some willingness to actually buy them. I passed on a Drum a long time ago because it was basically equal to my entire amount of spending money for the show. I just didn't want to walk in buy one card and leave. Not a smart move, but the tough backs hadn't really taken off then. The marginally tougher ones weren't even set aside as special.
So overall, look for the backs but don't get crazy about getting them all or sticking to one tougher back unless you've got the budget and some patience. Pick up cards you like that seem like good deals, and maybe opt for a tougher back now and then. Like "Do I get this card as a VG Piedmont or a G-Vg Old Mill"
Steve B