We've been MLK week a few times, and it is crowded. Before it would mostly be just the weekend and the holiday, but this year the entire week was crowded. With good planning it was no issue, but it was quite crowded. Small World had a wait of 45-60 minutes the couple afternoons we were there. I've never been there when it was that long a wait before.
We got to the parks before open and rode the rides that weren't quite as popular (we didn't go to FOP/7D/Toy Story land at open), and used FP for later in the day for the previous mentioned most popular ones. Also got in line within an hour of close for lower waits. This plan worked out great as we rode RNRC, TOT, BTMRR, Splash, EE, TT, FEA with very little waiting between early morning and late evening.