My guessing strategy for questions that I had to skip (due to time) was to simply have a pre-determined 'guess letter' that I always used. I have now adopted a new guessing strategy that has consistently earned me a point or two extra than the 'guess letter' strategy.
I am a very slow test taker so I have a lot of skipped questions (but high accuracy, so i score in the low to mid 160's). I usually have to skip 2-4 questions in each LR section, 2-3 questions in the RC section, and in the games section I always get 2 games perfectly w/ no errors but don't even get to the other 2 games.
On the LR section I always skip the parrallel the reasoning questions (cause they take longest) and maybe one more question near the end. Instead of bubbling in these questions as I go I leave them blank. Then, at the end with about 30 seconds remaining, I scan back over the ones I skipped and look for a letter in each case that is "yearning" to be bubbled in. For example, if an A is bubbled both before and after the one I skipped and there's been a long span of no C's around the one I skipped, I'm going to select the C. I do the same on the RC section.
On the games section, after completing the 2 games (i usually have 2-4 minutes left) I look and see what letter was used least often. I then use that as my guess letter for the remaining 2 games I had to skip.
Since adopting this approach I get way more than 20% of the ones I skip correct. Sometimes, like the last practice test I took, it really pays off - I got 6/15 skipped questions correct. Those 6 extra raw score points moved me from a 163 to a 168.
I know I won't always get a payoff like that, but I've definitely noticed an improvement in my "guess on skips" accuracy since adopting this approach. It's definitely above the 20% i'd expect from blind guessing.