Hi and thanks for your question.
There are NEVER forwards-compatability issues, but occasionally we introduce backwards-compatability issues (but that hasn't happened in over a year, and we still support iOS 5 + iPad 1, so there are no issues).
I would suggest NOT waiting to create stuff with 1.5.x. The changes you can make for 1.6 will be relatively trivial compared to the main job of creating your layout(s).
Hit us up again back here if you have further questions. Thx!
Comment 2014-12-26: This was a year ago, and the layouts created in a 1.3, 1.4, and 1.5 are still working in MIDI Designer 1.7.. and we're working hard with extensive regression testing to keep this true.