I tried that path (cleat covers), and it just is not worth the time and effort. I wear my cleats out on pavement, not walking around, so I never got any appreciable amount of extended wear from them.
I use both types of Shimano cleats, SPD (SH-51) and SPD and SPD SL, depending on what bike I'm riding. Generally the cleat covers are worthless, EXCEPT, if you're going to be stopping by someone's store, especially one with nice wood floors, either carry some cleat covers or throw some flip flops in the back of your jersey. I'm sure you wouldn't wear them on your own brand new hardwood flooring, and a shop owner probably doesn't appreciate it either. Many shoes that use regular SPD cleats, have recessed channels for the cleats. These used to be most common on MTB shoes, but are becoming more common on road shoes, also. These still don't mean you're 100% protected, but that nice antique shop owner that you had to stop at, might really appreciate your thoughts and kindness, concerning their nice shiny marble or tile floors. Other than that, most wear comes from regular use, not covering your cleats while walking. And if you can, just wear tennis shoes, if just running down to the local 24/7 store to get a coke or whatever.
Last, the price of cleat covers, in some stores, rival the cost of cleats themselves. I think of it this way, if you've truly gotten to the stage in your riding skills, that you're ready for the benefits of cleats, then they are expendable, like chain lube, or other maintenance items, and you just need to factor them into the cost of your sport.