I remember a discussion on types of borders not long ago and someone said that piano key borders tend to wave more than straight strips. At the time I had one under construction that was piano key, and I became pretty nervous about it. I was careful to be sure I had the same number of "keys" on each side and even did mitered corners that turned out well. I think quilters do have to be careful when they attach borders, but I would say don't be afraid to do what you want to do. Just for the record, I pinned mine to be sure my sides matched and that the same "keys" came together in the mitre.