Last disguise isn't as effective as you'd think, though; as soon as someone spots you in disguising in the killcam, they usually call out to their team, 'Spy is <whatever>'. Best bet for the 'b' key would be to last-disguise and run off, cloak, and take the time while you're invisible to pick your way through the disguise list.
What I'd prefer for the disguise interface, though, would be a one-key disguise select. Trying to disguise on the run pretty much limits me to the first three or four disguises, otherwise I have to take my hand off the WSAD keys or the mouse, both of which are counter-productive towards trying to flee. Also, hitting 4 then clicking is a bit icky, too, because you can't quick-switch to your disguise case. Not that I've noticed, at least. Maybe I'm doin it wrong :v