The question comes down to how happy would you be use your SNA for a Jr. Suite. If you would be happy knowing that 5 days in advance you could confirm a Jr. Suite and you would be satisfied with that, go for it.

It is really going to depend on the property. For instance at the Le Meridien San Francisco, if no suite is available you would get a Studio Suite. For me, I am happy with a studio suite and so I would not bother to use a suite night upgrade at all as I know that the category below a suite is almost certainly guaranteed, so why bother to use a SNA. For another property where there may only be a couple suites and Jr. Suites are hard to come by as well, I might go for the Jr. Suite if I really needed the room.

In the end it depends upon what other uses for SNAs you have for the rest of the year and what you would be happy doing.