I'm afraid I have to disagree with a couple of the previous answers. First, I am I said has apparently not had much experience with PLUMBING and it's components. When dealing with plumbing pipe, both old iron style pipe and copper pipe, the fittings for these, that use threads, are all tapered, and for good reason. Tapered threads can be tightened to eliminate leakage, sometimes even with a dent or two or three in the theads of the male pipe. However, these threads are meant to have a sealant of some sort used on the threads, (usually applied to the male's threads). This sealant can be EITHER as a paste, which is applied to the threads with the fingers or a small brush, generously, OR the teflon tape. The tape must be applied by wrapping the male end of the pipe, in the same direction of the threads, which is usually clockwise. This way when the pipe is introduced into the female fitting, the tape stays in place and actually pulls further down into the threads and DOES make a leakless seal in 98% of the joints on which it is used. Most of the other 2% of joints, the ones that leak, are found, upon inspection, to have had the tape installed incorrectly, so that it actually came out of the threads when tightened instead of pulling farther in. So, the leaks weren't caused because of the taper of the threaded ends.
As to the leak under the nipple connection of the hot side, this is a very ambiguous description, as there are many different fittings and pipes that can be connected to this point and without actual viewable evidence, it's virtually impossible for us to tell which connection you're speaking of. I have, however, installed many, many water heaters, both electric, gas and commercial units and of all of those, and considering the use of many different helper handles on the pipe wrenches, I've never managed to break a welded fitting loose from the top of the tank. I've also never had a tank leak from a weld done at the factory. These things are always tested under air pressure before leaving the factory. I'd say it's virtually impossible for you to break the weld away from the fitting on top of your heater. I usually check the general plumbing of the home when installing a new water heater, and if I don't find any, I install a cushion pipe in the hot side line before the entrance into the water heater. This not only provides the cushion for the water to expand and not blow open a T & P valve, but it also stops water hammer from occurring when the water is shut down.
And one last thing, I have found that it's very, very seldom that you need a 12" pipe wrench to properly tighten pipe fittings and it's NEVER necessary to use a helper handle on the wrench you are using, to tighten joints. It's sometimes necessary to use larger size wrenches to loosen fittings, but not to tighten them.