I have SPDs on my mountain bike and SPD-SLs on my road ones. I wouldn't use SPDs (mountain bike cleats) for a raod. The surface area is small (like a walnut) and feels uncomfortable for long rides. The larger surface/contact areas of the SPD-SLs is much better suited for road bikes - that's why Shimano makes the two types.

As far as keeping the tension loose, I also wouldn't do that. I've come unclipped twice - once when sprinting and another tile climbing a steep hill. You need enough tension to keep that from happening.

Despite all the posts about being afraid of using them, it's easy to get it down. If necessary, put your bike on a trainer and practice clipping and unclipping. Then go to a parking lot and practice. After a short time, it all becomes second nature.