Is it bad to take 1-2 Imitrex (sumatriptan) a week for tension headaches?

Headache log. I would suggest starting a headache log. When you get the headache, where it is, how severe, what you were doing before, etc and what you do to help it. Take this to your doctor. Tension headaches are due to referred pain from muscle tension. See a massage therapist who works with trigger points and look at clair davies' book the trigger point workbook. This may help you manage your headaches.
Are they Tension HA? Are your ha's really tension-based. Many so-called tension ha's are really migraine especially if they tend to throb, are accompanied by nausea, light & sound-sensativity and respond to imitrex (sumatriptan). Imitrex (sumatriptan) is pretty much migraine specific, although it will work to some extent on non-migraine ha's. Imitrex (sumatriptan) would be an extremely expensive way to treat a tension ha and does have some side-effects..
Treat cause. True migraine don't come on that often. You are probably treating the secondary migainous features that occur when primary ha/cervicogenic ha are not treated and they then trigger central features. Treat the underlying cause of your ha, like your neck, and you might not need so many dangerous triptans.