Remember everyone that we are working in a different realm when we work with hand tools. It’s much easier to swipe off a couple of thousandths off the board that’s fitting into the housing dado with a smoothing plane than alter the housing dado wall. And it’s a lot more effective than using a side rabbet plane. If its 1/16″ or more, then that would be different and I would alter the width of the housing dado.
If you just have to alter the housing dado width, I have used my square and knife to incrementally score a line where I desire my new knife wall to lie. Usually repetitive knife cuts along the straight edge will allow me to cut a few thousandths of an inch to widen the housing joint.
Another option is to pare the side wall a little wider with a chisel if the discrepancy isn’t too great, although I’d tend toward using a side rebate plane to trim a through joint. There’s no real need to go out and buy a side rebate plane for this kind of work unless you need to carry out a lot of deliberate trimming (Such as fine tuning the fit of sliding dovetails) and can justify the cost, as the simplest option (As Paul has already stated) is often the best course of action.