It didn't seem to matter with the ones I have. If you want them to be a tree shape, keep the lower growth trimmed off and just one cental leader. Let's see what some else says. I have 10 foot, at least trees.

I don't think it matters either, but I've never heard it before so wasn't paying attention. I do think how you prune it will be more important that where the original cut came from.

I could be that your brugs come from cuttings taken above the Y to begin with. You can train them to be taller "tree like" plants by doing as Kate and Katkin describe and picking a single leader and trimming off all lower growth and side shoots until they reach the height you want then letting them bush out on the top. Cuttings taken below the Y will grow up until they Y then they bloom creating a "tree" without all the trimming.

Cuttings taken above the Y will bloom sooner [sometimes ending up with blooms that lay on the ground under the plant] and cuttings taken below the Y take longer to bloom since they still have to Y before they bloom.

It also depends on the type of brug you have. Here where my brugs freeze to the ground every winter I have one that Y's at about 4 feet and starts blooming very early. But then I have another that doesn't Y until it is about 6 feet tall and doesn't start blooming until late summer.

I hope this has helped!
nova