Well, as I know, if u buy the guitar from Sg, and u bring along the receipt (or anything related that show u are buying from SG), definitely they will tax u.

So here's my suggestion (and I did b4 actually): I bought a bass guitar from Sg, and I just throw all those receipt bla bla bla, and the officer really got ask me, I told him I am a bass guitarist, and just back from a performance in Sg, and they just let me go. lol