The clipless setup I started with, and still use: Shimano PD-M324 pedals and the SH56 cleats on some SPD mountain bike shoes.

The pedals are clips on one side and regular pedals on the other side. The clip side tends to naturally rotate to the top, so I rarely have issues finding the clip. But this means I can consciously flip it over with my foot and pedal the old way if I want. (eg I'm stopped at one street crossing, and the next one is only a block away, I won't even bother clipping in. Or if I'm coming up to something where I'm not sure if I'll need to stop or not, I can get the unclipping out of the way early.)

The 56 cleats will come unclipped if you twist your foot in any direction, whereas the 51 cleats only work in the one direction. My belief at the time was that if I wrecked at any sort of speed, the 51 cleats would keep me clipped in, which would likely twist/injure something worse than if I was unclipped. I don't know how true that is, but I can confirm from first hand experience that the 56 cleats do indeed unclip themselves just fine if you were to, say, wedge your front tire into a crack in the road and go flying over the handlebars.

Also, having an indoor trainer was a great way to practice with them before going out into the real world.