Shimano Hollowtech II bottom bracket: milling the frame necessary? In a bike shop I recently asked for bottom brackets. The seller said, when choosing a Hollowtech II bottom bracket (like SM-BB70), compared to Octalink/square taper bottom brackets the outer (lateral) sides of the frame should be milled to be even. When they're not even I'll risk that the bearings will waer out too fast. He said even paint remainders are not allowed.

Is this right?

My frame is 7005 aluminium alloy.
The term for preparing the bottom bracket shell in this manner is called facing, and yes, it is necessary for all outboard bearing bottom brackets for proper function.

To understand why this is, lets compare a hollowtech type system to a square taper cartridge.

In the square taper, all the bearings and rotating portions are contained into a rigid metal casing, which is then screwed into one side of the bottom bracket shell, and a cup is threaded into the other side to hold the metal casing in place. If the bottom bracket shell's surfaces aren't perfectly flat to each other, it just means that the casing will be just a tiny little bit crooked with respect to the shell's optimal placement. However, all the bearings will be spinning around perfectly in the metal casing. At worst case, if the shell is very poor, there can be a little play between the casing and the cup, and the bottom bracket will creak when you pedal. An annoying problem of its own, but the bearings will be just fine.

On the other hand, with a hollowtech system, the two bearings aren't held together by the rigid metal casing. If the bottom bracket shell isn't faced properly, the two bearings won't exactly line up. Then, when you try to adjust the bottom bracket, one portion of the bearings will be too loose or too tight, and as the crank spins around the bearings will alternate between too loose or too tight, rather than being just right all the time. This will wear the bearings out faster and drag somewhat compared to a proper installation.

Now with all that said, it's not like the BB is going to spontaneously explode if you don't face it properly, you'll just wear the bearings out faster. If you already have a crankset laying around and you don't put a ton of miles on the bike, it might not really matter in the long run, but only you can decide that for yourself. If the frame is 20 years old, I would expect you to have a square taper crank on it already, and so there really isn't any benefit to switching to a hollowtech setup, in my opinion.