Do you think a person in a relationship should tell his/her partner if they're using sites the like of sw and others?

I guess it depends what you are using for. If you are using it to pick up other people, perhaps you need to ask yourself why? Especially if you already have a partner.

If it is just a place to write, if they seem interested in wanting to know other sides of you, sure, why not?

My thought on this is, if you are doing something you think you need to lie about, why are you doing it?