True, it's not rocket science, and I may have tried to write a 'dummies guide to....' when that was not called for. But as pointed out before a good read of the POH would have answered your question and personally I think it is compulsory reading before setting foot in an aircraft. If the club doesn't let you, then hire somewhere else.
Going back to your original question, indeed you might just as well cruise at 100 kt instead of 90 without doing harm to the aircraft. As you say it is easier to not maximise your fuel efficiency. Two things though: 1. the power settings in that table are those at which, or between which you can/may cruise. That is useful information which defines your playing field. 2. Operating an aircraft efficiently shows good airmanship. That's never a bad thing. You never know when you might find yourself in a situation where you'll need those last drops of fuel and then you'll be glad you saved them.
Edit: Over here the POH is part of the ship's papers, you are required by law to have it on board. I don't have time now to refresh my memory as I cannot remember if that is EU law or Dutch law. Someone with more knowledge than me of UK air law will surely answer that question.