Agreeing that you could cap at any level you so chose...as long as your players were cool with it.
Personally, were I to run a 4Ed campaign with a cap, I'd cap just below Epic. But that's me- I like the way Paragon plays out, some of the new powers & feats sing to me.
Yes, personally if I were going to cap 4e it'd be at 20th. I think though that you can just run Epic as "extended Paragon", a lot of DMs seem to do that - Paragon style play just with bigger numbers.