Ken,

Not really.

With square rule you are truing the timbers at the joints to make them perfectly dimensional and square.

Now if you have the saw mill or planer mill do this work for you by accurately sawing or sizing with a planer and you either use timber free from or assume your timber is free from wind and bow and un-square faces, then that can save you a bit of work as you don't have to size and square up the timbers at each half of each joint. That's mill rule.

Both rely on perfect geometry at the joints, the difference is whether that is accomplished by milling the whole stick perfectly or by perfectly cutting reductions, gains, and housings at the joints.

Griffon,

As for your question. There is not a simple answer. Some would say yes, some would say no.

For me to mill rule, the timbers need to be better than 1/16 of dimension and 1/16 of square (on an 8 or 10" face). Just as critically, you must have minimal sweep and no wind. And you always need to adjust things with mill rule when you are putting them together -- you just haven't accounted for irregularities up front, so you must pay at assembly.

The short answer is that if 2mm is good enough for you on this particular project, then mill rule is fine. (No value judgment intended, everyone has their own tolerances and values.)

Mill rule is fast, I will say that.

GH