is it ok to smile?

Smile at everyone. Make their day. Just don't let it become a stare. Saying hello is really nice. It gets people out of their pity party.

i smile everyday, sugar :-)

Yes, most people will smile and say hello back, just avoid doing it to sketchy looking people.

I walk a great deal.  Often when I'm walking I pass people going in the opposite direction.  I often say hello and smile and usually get a smile and a greeting back.  A simple hello and smile can be nice, for the other person as well as yourself.  When I'm in a store or bank, etc. I always end the transaction with "have a nice day". and a smile.  I've never had a problem with that either.  I think it's how you say it.  And I don't stare.  It's called being friendly but not too familiar.  By the way, there are some  cultures where this behavior is almost mandatory.