I try to center the biscuit in the thickness of the material if doing a single row. On thicker stock I’ve done a double row and tried to center the cuts on thirds. As far as spacing, I generally put a biscuit every 4" or so being very mindful of the location of the finished end. Nothing will spoil a nice top quicker than an exposed biscuit on the end.
I have also found some variations in biscuits that can result in less than desirable results. I only buy Porter Cable biscuits. I bought some at Harbor Freight and promptly threw them away.
Artisan Woodworks of Texas- www.awwtx.com