I've tried many combinations:

1.6 all around
1.7 all around
1.6 tapered all around
3x1.4 tapered all around
1.7 front 1.6 rear
3x1.4 FLAT all around

My findings are pretty much what cherry said. The tapered 3x1.4 definitely lose grip in the rear. Only liked these on really high bite indoor.

1.7 flat with heavy oil is about my favorite for most cases.

1.6 flat really good as well... Especially better if you have tough landings for jumps.

I just yesterday tried out the 3X1.4 FLAT machined pistons... I ran them back to back with flat 1.6's... Seemed pretty good all around. Maybe a little sketchy on the high speed part of the track. We were just practicing for our ROAR region race next weekend. Here's a vid of the track:

Tires were JC green 3D's/barcodes... Dry slight groove

http://m.youtube.com/watch?v=zkhc1PCAC3w