I have had a lot of success with all female tanks and actually prefer them. I have frequently kept 2 females with no problems, 2 males always seems a lot dodgier.
Yes, you may still get fry, but they supply will dry out eventually. It is true that they can store sperm for up to 7 pregnancies, doesn't necessarily mean they all will, or that all those 7 pregnancies will be in your tank. In the last 2 years, I have bought 2 pairs of female guppies. Out of the first pair, one had babies once and then dried up- lived in my tank for another year without further pregnancies, the other had two pregnancies and then dried up. The second pair I bought (March last year) seem to have had one pregnancy each, and have not given birth since May, but are in excellent health and seem very happy with their celibate existence. One of my platies admittedly was a little more prolific...
When I kept all 4 common livebearers as a teenager, I had males in the tanks, so that doesn't count. Obviously, if they have a chance to keep on doing it, they are going to keep on doing it.
During this time, I have had different combos of female platies and guppies living together in different tanks- from large groups of 10 and more, down to 1 platy with 1 guppy, and several times I have had pairs or small groups of one or the other; only once have I had trouble with bullying, and that was one elderly fish who got very grumpy in her dotage. With males I have had a lot more problems with bullying- to the point where fish have actually died from stress. They seem to more prone to collapse at the slightest sign of stress, anyway+ seem generally weak-minded. Females seem more outgoing and less prone to stress. Just my opinion.