I think that you'll hit a point pretty soon where you'll have to change squats to twice a week rather than three anyway. I.E. when you hit a wall and do a back off you'll probably need to switch it over at that point. When I say twice a week I mean you will actually progress twice a week. I would wait until that point before adding a set to deadlifts and then you will simply need to see how it goes.
Another thing that is easily forgotten is when deadlifting becomes difficult to add to you can use adding a set to the same weight as a way to progress. That is, you would do 1x5 at a certain weight and then attempt to add a set to it before again adding weight and doing 1x5 again. This will do a lot more than you think but the question is whether you would get to that point in deads before you're on to something else entirely.
I don't know all the ins and outs of what Rip would recommend to any one individual (maybe Dave does) but in his writings he usually coincides the adding of extra sets to deads with the encorporation of power cleans (at around the time when squats go to twice a week) so that deads are alternated with power cleans. My personal preference would be to keep the deads the same and add a set if possible and save the powercleans for the point at which deads become too difficult since the cleans themselves would be a great dead assister.
Remember that Rips main business is the training of athletes and some a lot of his advice is generalized to suit that. That means it may or may not be apporpriate for any one individual looking to build pure strength. My instinct is to say that if you are progressing on any exercise in a certain way then keep on doing that! You don't fix what ain't broken. The way you are going your deads are getting really strong and there is not particular advantage to adding volume at this stage of the game when you are loading the bar on every dead session.
Just be flexible. It's not going to kill you to try something to see if it works or if it's a detriment to progress. The key is just having patience in the matter.
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