I don't think RSS helps rankings directly. The only site I added an RSS feed to (which gets updated very regularly) didn't see any appreciable increase in traffic. If you have good content that making that content more easily available (e.g., through RSS) could mean more readers which could mean more backlinks if they like your content, but I wouldn't expect adding RSS in and of itself to increase rankings.
My general advice is always that if you want good rankings, make your site deserve the rankings, and the rankings will follow. If your site offers info, make it awesome info and easily accessible. If it's a store, be the best at something among all your competitors (price, quality, service, easy of ordering -- pick one or more).