I used to do seasonally too, but there has been more and more cases where I live and in areas that it wouldn't be expected, so I now do year round also.
I'm probably just get her tested soon then and start her up again right away. In your average normal year here there really are many months where there isn't much point because it's far too cold for insects, much less mosquitos actually carrying heartworms. This has definitely not been a typical year, though so I guess I'll just play it safe the best I can.