Personally I don't think it's necessary to wait until it's r50 to train. Just only train in stats that you intend to max, because otherwise you might end up with some stats higher than you intended. I do like to be at least 20 ranks behind in training as I'm approaching r50 though.

Also, if you absolutely need a particular stat at 500, don't wait until r50 to do it since food will increase your attributes by 15, and you might end up at r50 with an attribute at 490 or 495. If you feed beyond that, you'll be losing attributes. So as you're getting close to r50, try to catch the stat with a multiple of 15 left, and train it right up to 500.

Be careful! be methodical! Know what stats you want! Make sure you don't have any grade 1 food in your inventory or you might accidentally feed it! You only get one chance!