I used to do them with every back workout. (once a week)
But I found that sometimes my lower back was never really 100% recovered. It felt great before hand, but once I got into the workout I could tell. I found that it was tightening up really bad on leg day, (squats) and when back day rolled around again, whoa! It was VERY tight and it was distracting me so much that my workout was suffering.
Now I hit deads every second back workout. I feel a lot better on leg day and especially on back day. My back doesn't tighten up to the point where I'm in pain aymore, and I can train with a lot more intensity.
I think what it comes down to is personal preference though. Not everyone is the same. Some can hit deads every week, while others might hit them every second week or maybe once a month.......who knows.
If you feel like you are fully recovered when you do deads every week, and you can train with max intensity and are still making good progress, then go for it!
Just listen to your body and you should be o.k.