Matters a lot on a sickle mower.
Not as much on a mower conditioner with a reel to clear the hay.
Top serrated will plug up a sickle mower with fine stemmed grass. Many sickle mowers work best with smooth sickles and serrated ledger plates.
But with a reel I most often see smooth guards and top serrated sickles nowadays. Bottom serrated used to be more common, as those could be sharpened a time or two. We aren't as frugal any more.......