I don't know if it would be an issue or not - I just recall a comment or two about the 990's second trigger bouncing off and back on with input changes, but that might not be enough to affect the SMS-1.

For what it's worth, I just leave my SMS-1 on all the time - haven't turned it off that I recall since setting it up over a year and a half ago. It doesn't draw much power, runs cool, and has worked just fine being on continuously.