Originally Posted by day_trippr
Pretty much depends on the individual pipe run; sometimes you can get away without a union, other times it's a sanity-preserver to have one...
A picture or diagram might help with a recommendation.
On rare occasions you can thread an entire assembly together and mount on your brew stand, or whatever.
More often though, you are left with pieces that have to be attached after something else is already mounted in place or there are clearance problems that prevent rotating the pipe or fitting..
The union in the answer to this problem.