Yankees are only worth doing if youre backing short prices. No point in having a 20/1 winner on yankee and getting fuck all back. I do 31s and 63s as on a saturday i probably back around 30-40 horses. On most 63s i would lose only having 1 winner but if you have 5 63s all with one 5/1 winner on you would win more than having 5 heinz's at 5.70.
Also betfred pays 4x the odds for one winner on 31s and 5x the odds for 1 winner on 63s so if you get a 10/1 winner on a 63 you have a 5a back which isnt that bad.
Each to their own but 15, 31s and 63s are good bets if you are backing a lot of horses rather than yankees, canadians and heinzs.