I usually don't do it with personal presets, but I have sometimes tried to make a discreet watermark somewhere on the preset! It's a bit difficult to find somewhere that won't be too distracting, usually on a paw pad or on the stomach work alright.. otherwise you can always put a watermark in an unused portion of the UVs :o
Especially for commissions, I think a watermark would be a pretty good idea, I wouldn't say its weird at all!