It depends on the strings and frame in question.
Let's assume a softer main (gut or a soft multi) and a stiffer cross (poly or a stiff multi or synth gut). Generally with this setup you'd want a lower tension cross and higher tension main to balance the two. You probably wouldn't want to string the mains looser in this case.
Reverse the above and you might want to string the stiff mains looser and the soft crosses tighter or the same since the natural differences in stiffness might balance the two.
In the two cases above you don't have much of a choice and spin will be determined by other factors.
Assuming a homogeneous setup you generally want the longer mains strung a little bit tighter than the shorter crosses to even out stringbed stiffness. The idea is that the longer mains naturally flex more than the shorter crosses and in the interest of producing a reliable SB you might want to balance the relative performance.
So again, there's not much of a choice in some ways.
The problem is that not paying attention to string choice and pattern first can result in a wonky stringbed. For example, string natural gut mains at 48 and super stiff poly crosses at 52 in a very open pattern and you'll probably have some control issues. Spin will be the last of your worries.
To maximize spin potential start with your choice of mains and crosses, pay attention to string friction and ball friction, and then base relative tension on string choice and density (more open string closer together, more dense you can increase the difference). Worrying about spin potential and relative tension before considering string choice and pattern is putting the cart before the horse.
Finally, the mains do contribute most to spin but the must be just right, not too soft (won't snap back fast to flick the ball and cause spin) and not too stiff (fail to move enough). I've experienced both using the same strings. For every string choice and frame there's a Goldilocks Tension for spin, power, and control.
2x BLX 6.1 Pro Staff 95: 12 oz., 8 pts. HL, SW 325
VS Touch Black/Focus Hex 1.23 @ 58/54
Last edited by TimothyO : 11-18-2012 at 04:27 PM.