Volvo seems to have no position on this, and there is no key way or other mark to go by on the props or the shafts. My research found some reasons for clocking them so both blades do not pass under the anit-cav plate at the same time. There is no downside and it is simple to do. Put the big prop on with a blade straight down, and the small prop on with one blade straight up. They turn at the same rpm so they will always pass like that under the plate.
Here is a sample of what I have found, there is more out there and one time I even found a Volvo tech that thought this made a difference, but this is a good microcosm of the debate...:
Yep, even Mercruiser doesn't say you have to time them, but if you have them off already, there is no reason not to. It's easy to do and eliminates any potential issues.
