You need to tell her about this in two instances.
If you are about to get it on and she asks of you have a condom/or hands you one AND you already know you would bare back this chick. Then offer to go without one letting her know that there is no risk of pregnancy.
The other time is of she brings up wanting kids and/or asks you of you want to have kids. Tell her then.
Or if she asks directly, but that rarely happens without some sort of indication.
Otherwise there's no other social reason for it to be discussed, unless you just like putting it out there.
As far as lowering your SMV, if she does not want to have kids then it makes you more attractive.