lots of very good info in the archives on this topic...
enough to sort out the basics for almost ne1.
with a tongue weight significantly UNDER 500 lbs many suggest no w/d is needed.
IF w/d is used the bars should be rated ~the same as the tongue load.
so 500 needs 400-700 lb bars.
depending on TYPE of sway control design
built into the hitch (cams for example) ,
the w/d bars NEED to flex a given amount for the sway function to work,
and higher rated bars may not flex enough on lighter trailers, so the sway control isn't active.
lighter bars are useful with beefier rigs and LIGHT trailers, sometimes.
a simple outrigger adjustable friction sway control bar (or 2 of them)
is often used with small trailer/lighter trailers...
where w/d is NOT needed but some added stability is desired.
these threads include great depth and debate on the basics and useful to read...
scanning the titles in the hitch section, one will find many others that are specific for BRAND or setup of hitch, and useful reading.