I agree tables should be learned in 3rd grade, but if that's not the case I think 1-5 minute drill will not kill anybody.
As for skipping 5-8, I would probably cover all the areas he may have not covered yet before jumping to algebra. I would make sure he can add/subtract/multiply fractions, calculate %, area and volume of different shapes, rounding, angles. That's up to Singapore 6B, I don't have anything beyond that. I would also go through some challenging middle school problems (Singapore Challenge problems, Zaccaro middle school, Mathematical Olympiads for Elementary and Middle schools).
DS5 does 4th grade math, but I still feel there are lots of things to cover before he starts algebra. The good news is that he is finally getting to the level when challenging problems can require some effort. There is only that much you can do in the first few years, but you can come up with interesting problems for the middle school level.
You are all right that lots of things repeat or are very obvious and the kids don't really need to learn them, but there are still new things to learn there.
