Count me among those who say you absolutely positively MUST measure a chain long to loop the big/big combination with an inch to spare. The potential consequences of accidentally shifting into a combination the chain cannot loop are too severe to be acceptable. Big/big + 1" is the minimum chain length, though I prefer to run chains longer than that.

As to whether you ever use big/big, that's up to you. I don't consider it s big a problem as others do as long as it isn't a steady diet.

If you're limited in RD capacity, measure the chain for big/big, and let it be too long for small/small. Worst that can happen is that it'll sag, but it won't wreck a rear hub or derailleur.