You're doing a scarf? Personally, I do first/last st on WS but some prefer to do first st on both RS and WS, others do last st on each side. If it's NOT a scarf, or similar, and the item is to be seamed, I'd not recommend slipping as that size st doesn't seam as well (doesn't provide much stability).
As for number of sts, the slipped st gets worked on the side where it's not slipped (for instance, if slipped on the K side it's worked on P side). No loss or gain.