Tacky in the extreme. Actually, I was always taught that a gift was never required when invited to a graduation, shower or wedding, just a nice card.
After going through my DSS's high school graduation (small town, everyone knows everyone) this past June, I'm starting to feel like invitations are a blatant bid for money/gift cards/gifts. Why would they send me a invitation to their graduation when they know I'll be there for the DSS and I'll be at his party, not theirs!!!???