In a stitch like moss/seed, garter or ribbing which is more or less reversible, take a look and decide which side might look a little better. Sometime the CO edge looks better on one side. Then choose that for the RS. A lot of times when "end with a RS/WS row" is given and you've been doing a reversible stitch, it may be you're going to switch to stockinette or cables where there is a real RS and WS so you know you'll be starting that stitch on the right row.
Personally, I dislike when they say 'end with RS/WS facing" because I don't know if that's before or after I turn for the next row. End with WS/RS row is a lot more clear - finish that row and you start the next section on the following row.