We've built several miles of fully electrified hi-tensile woven wire fence all on steel pipe end and corner assemblies with PowerFlex posts for line posts. I would say from our experience it is better to tie off at every 90-degree corner. About 120-degrees is as tight a bend as we like to make without tying off.
In your particular situation where you are making a lot (or that's the way it sounds), I think you might be able to go around a 90-degree corner, assuming it is a very well constructed corner, by using Gripple connectors or in-line strainers on both sides of the corner or at each end assembly.
Let's say your fence was going to be about 200 ft on each leg of the L. Use Gripples at each end of the fence so that you could tighten it from both directions. If you were planning on using standard in-line strainers, I would do the same thing. If your runs were longer than 200 ft on either side, I would use the Gripples or in-line strainer but I would install them about 30 ft to either side of the 90-degree corner with the pull towards the end assembly.