Testing them once per year is a GREAT idea - IMO. That said, when i owned an ABS Escape where the canister and trigger cost 170 each time I fire them, i did not test on an annual basis. Now that I have a BCA with normal air in it, and i have refilled with a hand pump (so I know it can be done the hard way - and it's not all that bad) - i will test mine at the beginning of each year.
Ultimately, it's my own piece of mind that comes from testing it. Regular testing doesn't make it (or me) invincible - i still need to make smart decisions, but it sure adds a little bit of extra comfort knowing that it will work if needed.