JW- Normally I would be inclined to agree, in this situation its still iffy IMO. Veteran pitchers do use spring training to do as you stated, however, 95% of the time that pitcher has been in that role in the recent past, not the case with Graves or with Lowe last season. Spring stats in this case could very well give more insight into what to expect, at least early on in the season.
With that said, I believe Graves will begin the season in the rotation and end it there as well. He's a good pitcher and he knows how to win games and has the mentality to be really good as a starter.