Are flash cards a no no? We are currently using Saxon 1 for my daughter and she drills her math facts as they are introduced in the text. We started with doubles, then +1, then -1, plus zero was in there somewhere and we just added the plus and minus twos a few weeks ago. I have her review them as part of her memory work and I check her on them on Friday. She also has a 25 question math fact review each day as part of her lesson. I wasn't really sure she needed that level of review (and she probably doesn't), but she can go through her entire set of flashcards in less than 3 minutes, so she must be doing something right.
I'm not sure if the memorization would help with the Singapore Math, but I can already see the benefit in both my first and second grader. Also, mom is getting a little faster at the mental math LOL!