yup, if you are developing i would pick up as many as i can.
if you are trying to sell it, then picking up all the other extensions isn't as important.
it should also be if you plan on developing it into a large site commercial or otherwise, if its gonna stay as a small site or more personal, then i don't think you really need to pick up all TLDs
Well ofcourse its great to take the other primary extensions.
But then it also depends on how much investment you are ready to make; and for what purpose are you buying the domains.
If its just to flip and you are making your portfolio, and you decide to go for like 4 exts per domains, that 4 times the costs and 4 times the renewals. If you build a portfolio of 100 domains, with the set of 4 you will have to retain 400. That kinda sounds crazy.
What i would suggest is, only keep the sets of domains for :
1. Ones you wish to develop and market.
2. Really Good generics / dictionary words (lolz if you find any) or other highly premium domains.
4 times a garbage domain is still garbage, so go for the good ones in sets.
just my opinion