Lying
If you go backstage after a concert, you compliment the performer. If this requires lying, you lie. If you can’t compliment honestly, and can’t lie, don’t go backstage. (You can always say, with a sincere look, “Thank you so much”; do not add, “for your hard work.”)
Compliments
They have two parts, one essential, the other optional. The essential is your personal reaction. (“What a treat!” “I was swept away.”) It can even be ambiguous (“I don’t know what to say!”); but if this is a way of finessing that you didn’t like it, be sure no hint of that comes through. Remember that hiding things is tough; for as you’ve just been reminded by the performance, “Everything is obvious.”
The optional is something specific (“You had that piano in overload.” “I’ve studied that fugue, but you showed me how a Bach fugue should be played.” “Your chord-voicing was a revelation.”)
You don’t have to be knowledgeable. One of the compliments I treasure most came from a lady who simply said, “Your Schubert slow movement took me up to Heaven!”; another came from a touring virtuoso who said of another program, “Every note was in its proper place.” (In this case, it wasn’t so much the words as whose mouth they came from.) And never fake what you don’t know. Performers are professional receivers of comments, and to fake musical knowledge with them is like faking the Second Law of Thermodynamics with a Caltech class.)
Ownership
Bear in mind that your compliment gives you no ownership stake in performer or performance. Do not say: “Your Mozart was very good. I couldn’t help thinking that playing in time would have made it even better.” Nor, “I’d like to talk to you seriously about your Schubert.” Nor, “The Moonlight Sonata was very good, very good, but not like Horowitz.” I admire the flutist who was greeted backstage by a complete stranger who leaned in and said, “Your Bach interpretation was all wrong. We should discuss it.” She smiled brilliantly and said a warm, “Thank you” while using one hand to “swim” past him to the next person in line.
So, yes, do go backstage and say a word; you’ll help the performer through another day of an arduous life. But keep in mind that he or she may be exhausted physically, mentally, emotionally and spiritually. Don’t take apparent or actual lack of attention to you and your words as disdain, condescension, or rudeness. The performer’s tired! 
