I saw an orchestral member smile during a performance. Not once, twice or three times...rather many times over the length of a concert. Sometimes it was at the conductor, at the concertmaster, or her stand partner. It was great to see, and makes a huge difference. The only disappointment was that few of her cohorts did the same. And that's true often of musicians.
I'm not saying you have to grin to make notes, or to express the phrase, that's a whole other technical matter. But to convey you're enjoying what you do? Absolutely! Too often performers look lackluster or nonchalant when it comes to performing or receiving applause...again, I'm not saying smiling while playing a Bruckner adagio is necessary (or appropriate) however, after a performance, or while making music, sure, it's alright to smile - enjoy yourself.
I made sure to catch the performer last night, and say what a pretty smile she had, and I was glad to see an orchestral performer smile. Sure there was a bit of flirting, but the message of smiling came through.
This does lead to all of us, you should smile as often as you can. There's the old line, "smile, it'll make people wonder what you're up to" - and I can always hear when a radio announcer is smiling. I try to smile on air and off air.
Pass it along.