It depends really. A lot of 7 strings now are 27ish, Ibanez being the big trend bucker (except on it's Korn models). A B string is a B string at the end of the day, wether it's 6 or 7 strings, so it becomes something of a trade off, stability of the B string Vs bending of the upper strings, although of course the increased length means you could put very light unwound strings on with little difficulty. I personally don't feel a huge difference between 24.75 and 25.5 when it comes to bending, so I'm hoping 27 won't be a problem.