This is a great question Stephanie! I don't think there is a definite yes or no answer, but here are my thoughts.
If you are invited to the engagement party, all that is required is a card! That's it, unless you are the parents, grandparents or sibling, then a little more may be required.
If you are invited to the bachelorette party, I don't believe a gift is necessary, but usually a gag gift of some sort, OR you can assist in bringing something necessary for the night (ie: drinks, food, etc.)
For the wedding, it is ok, to again, buy off the registry or simply give a gift card or money.
I guarantee in 10 years (heck, maybe even 2), the bride and groom will not even remember what they received as gifts from the showers or wedding.
Just remember, give only with in your means. Do not go into debt supplying the bride and groom with the top of the line blender. Only buy what you can afford.
I hope this helps! Let me know.