We don't need to "keep" Stassi. Even in the tiny chance he got selected in the rule 5 draft his contributions are easily replaceable. Left handed first basemen with an 800 AAA OPS are pretty easy to find, especially if you are looking at older prospects age 26-30. We should not even think of protecting him. If he takes a step forward and wins a bench job at some point next year that is great. There is just no need to protect him for that to happen.
There is a better case to keep Asche. He can play 4-5 positions even if he is not great at any of them. And he has been a better hitter in AAA than Stassi (850 OPS mostly at younger ages compared to 806 for Stassi and Asche is STILL a year younger).
He is not a great bench piece, but he is not an awful one either. We need a backup middle infielder and a catcher. For the 3 other spots it would not be the worst thing to have Asche, a LF/1B type that bats left handed (could even be Stassi) plus a good defensive OF (could be Quinn as part of a rotation/platoon). I won't cry over losing Asche because he is also replaceable. I am just not sure we will find much better among free agents that would sign for less than $5 million/year.
This constant love of overage 1B in the minors is like the love of backup quarterbacks. They put up great stats against weaker or younger competition and really are not that good.
The big decisions on the rule 5 will be more among our AA and High A level prospects. Tirado, Tocci, Canelo, Anderson, etc. These guys all have decent ceilings and are young enough to improve. They are a stretch to stick on a major league roster though. With Tirado, for instance, it would be a huge benefit to slip him through so they would not use an option year. A guy like him could take a few years to figure things out so he might need as many option years as we can get him.