Dancechick-
I know it seems crazy to plan this far in advance, but if there is someplace you really want to eat at in WDW, its nice to make sure you have a PS.
I am sure you wont have much trouble getting a Doanld's PS (if you were wanting at PS at Cinderella's castle, then you you better be calling at 7:00am sharp 60 days out!!)
I am not sure how many Priority Seatings Disney has allotted for a certain day at a certain time.
Go ahead and call, if you have to cancel there is no penalty (at least not for Donalds) If you cant get a PS because it is full, keep calling every so often and you might get someone else's cancellation.
I called this morning for a PS at the Norway Princess breakfast. I didnt call until 7:30am (Florida time) because I had the impression that it was a not difficult PS to obtain (at least not compred to others) but when I called they only had two seatings left 8:50 or 9:50. I was figuring I would have more of a choice than that.
Good luck and have fun planning!