With DWV in evidence I would not consider going TF ... the first thing before you even think about TF is to do regular testing for varroa ... so many factors are involved in being able to have healthy and thriving colonies that can survive when there is varroa present (and there will always be varroa present). You have to check the mite levels regularly and only consider being TF if and when you are confident that your colonies, in your location, with your beekeeping, have maintained low levels of varroa, consistently for a significant period. Be prepared to treat if mite levels seem to be escalating out of control - it's a hard road to follow, there are no silver bullets and you risk losing colonies if the mite load is too much for them. Not something to dive into and hope for the best ...