I heard contrasting opinions about this as well - the main basis against this (for ukes w/out truss rods) I think would be that it resets the tension on the neck, allowing the neck wood to relax and bend back, but then once you put new strings on, the neck is tense once more. Considering how the neck doesn't move much, I don't think it's much cause for concern until maybe 40-50 years later doing this, but even then, I'm not sure that this affects structural integrity that much.
As for me personally, I like to clean the whole uke when I change strings (which is less than a year each time for sure), because I know how much oil and gunk sticks onto my fretboards, and how much I let other people play my ukes haha. So far, I haven't seen anything go wrong with my ukes, but I think if you're doing this once a year, you should be perfectly fine. If you have a truss rod in your uke, you're even more secure!
I've never heard of an uke breaking at the neck because the strings were changed too often like this... hopefully we never will! xD