From what I've been told, decreasing tension on the crosses does make the sweet spot bigger and increases dwell time on the string thereby enhancing spin potential.
Personally, I like the feel and drop tension by a couple of pounds. I'm currently using natural gut mains and poly crosses at 53/51
Wilson Six.One 95 16X18 Babolat Syn Gut 17 @ 55
Strung Weight 358g 359g Balance 31cm 31cm