There are a few factors I consider. I have a V8 with 4x5 and 5x7 reducing backs as well as a dorff 5x7 with 4x5 back. 4x5 and 8x10 have the same aspect ratio so I have to consider two things: 1) whether I want to enlarge (I don't have an 8x10 enlarger); 2) Whether DOF will be an issue. Sometimes with architecture, I'll take the 4x5 back in case I get into a DOF pinch. I have lenses for both so that isn't an issue for me except for weight.

The Deardorff splitters are a nice, cheap, and less heavy option. You can either shoot two 4x10's, two 5x8's or four 4x5's on one sheet of 8x10 film.

Usually, I either decide ahead of time which I will use and leave the backs at home. Occasionally, I'll take the back(s) and other gear (holders, lenses, etc) and just leave them in the car in case I need them.

If I had it to do over again, I'd use the splitters for reducing 8x10 in the rare instance I'd prefer 4x5, but buy a 5x7 back and take it with me. This might result in 1 additional lens and a few extra holders.