I'd like to add that if the person you are linking to is a long-time friend and you know the person will say yes (I have a link to my downline's website and didn't ask permission) then it is fine.
However, if the person you are linking to is known to you on a professional level (another stamping business owner included) then it is customary to ask.
You shouldn't assume that they will be happy to be affiliated to you.
"They're only crayons. You didn't fear them in kindergarten, why fear them now?" ~ Hugh MacLeod