I studied adult beginner's violin from a Chinese woman who had earned her performance masters degree from Julliard.

She consistently sang: si do; never ti do.

At first I thought I was just misunderstanding her accent, but then I asked her about it (rude little thing that I am), and she was very clear: "Si Do."

Interesting, don't you think? Every voice class I've ever taken, we used ti do. So, I don't know where the division of schools of singing has occurred or why.