You see the trick here, that's 99.95% outside of their regularly scheduled maintenance!
Oh! Now I see. I haven't used RDS for anything critical yet but thought it'd be a no-brainer.
insert mind expanding gif
In practice it works just fine. They just make you give them permission to blow your app up for twenty minutes a week (and wink and say maybe they won't actually do it this week).
With respect to SLA's the two important things in my opinion are:
* How precisely is it measured?
* What happens if it is not met?
As observed by the other posters, scheduled maintenance doesn't count.