SMC Specification sheet for the series of solenoids you are using:
http://content2.smcetech.com/pdf/SY3.5.7.9000.pdf
It looks like each solenoid will use at most 0.45 watts. I don't think keeping the solenoid powered will notably reduce the lifespan of the solenoid valve.
We exclusively use single action solenoids and leave them powered for significant portions of the match. We have observed that when hitting a defense hard it will momentarily pulse the solenoid. I suspect that it might actually be hitting it with enough force to pop the spool back, but I haven't done any tests to confirm. According to the spec sheet they're rated for impact (maybe they meant shock?) at 150m/s^2.