Looking through the list of P&G brands, I can say that I haven't bought anything from them in years, even when I lived in the US.
I do agree though that boycotting every company that engages in questionable behavior would be near impossible. It's better to change the system, but if in the meantime you can avoid giving money to the most heinous companies then why not?
Hint: You've already said it.
If you buy store generic-equivalents, its quite likely your money is going to exactly the same major corporation as the name brand in pretty much the same quantity (after deducting that the part that the major corporation would be paying to marketing/advertising firms to market the name brand.)
So, while it saves you money, it may not be as effective a boycott technique as you think. Unless your boycott target is the advertising industry.