It's a good assumption, but certainly not one to rely upon totally. The general consensus here (and I share this view) is that the B Galleries lounge is the best of the business class lounges, and if you know about the passageways, fully mobile and not overly burdened with handluggage then you can actually get from that lounge to any gate in T5 within about 10 minutes walk. So if you arrive in good time, you can head there straight up and even if it turns out to be T5C or T5A (probably A10 though) then you're fine to go there instead. All lounges have identical breakfast options, but get there before 09:30 hrs if you want bacon sandwiches.

The transit and walking are about the same time to T5B, allow 10 minutes from T5A. The passageway will win overall in any test, due to the fact that there is no waiting, but on the day a transit may be close to departure. The transit time is only about 2 minutes, if that, but even if you see a transit pulling in, it takes a few minutes to turn around. T5C generally the transit is best, it's quite a lot further, unless you actively want to walk it, which I generally do.