What do you think about me addressing a store employee with "yo"?
Why is it important to say that?
Because I want to sound cool
If you think that'll make you look cool, I say go for it!  As long as it's a friendly yo, you should be fine.  It's totally normal to want to look cool to other people.
Looking cool to others isn't relevant to me, just sounding cool.
Fair enough, whatever floats your boat. Attempting to sound cool is perfectly okay.