Starting in the West, I'd base my start time on when McKenzie Pass looks like it will open the year you plan to go. That varies from year to year, but I definitely wouldn't figure on earlier than June unless you know that it is a light snow year and the pass will be open. It isn't too unusual for the opening date to be in July, sometimes even late July.
It is generally open to bicycles a couple weeks before it is open to cars and that is a great time to do it. They plow out 1 lane and let melting take care of the other to save money. Once 1 lane is open it can be ridden by bike.
Santiam Pass is a plowed year round alternate, but it would be a shame to miss McKenzie Pass. It is a nice ride with great views and the lava fields and observatory on the top are pretty interesting.
As John said, if you must go early go E-W.
To get a better idea of what is likely check out past opening and closing dates at:http://www.oregon.gov/ODOT/HWY/REGION4/pages/mckenziehighwayclosure.aspx