One rule of thumb I've always lived by as a mechanic is - Always have "more" than you need...
I have a set of everything in SAE....1/4 short, 1/4 deep, 3/8 short, 3/8 deep...etc. You get the picture. I probably have 4-5 sets of SAE sockets that have literally never been put on a fastener, but I like knowing they're there just in case I need them.
If you work on newer vehicles, the chances are very slim on running into many SAE fasteners anymore. If you work on old stuff, it's absolutely necessary to have SAE everything.