In Axis and Allies, Does It Make Sense For Japan to Attack Sinkiang on the First Turn?
No! Japan should do no such thing. Here's my opinion of the right Japanese first moves (assuming no 1st rd. attack by Russia):

1) Assuming UK has built an IC on India:

Japan Build: 2 Transports, 3 Infantry

Japan Attack: Pearl Harbor 2 Battleships, 1 Sub, 1 Aircraft Carrier, 1 Fighter, 1 Bomber

The battleships can't free-shot since Manchuria hasn't been taken, thus send them both to Pearl Harbor.  You should win this battle round 1 and be left with the Battleships, Aircraft Carrier and Fighter.  This ensures America uses its Battleship/Transport/1 Bomber and 1-2 Fighters on its turn to wipe you out (a great trade to keep US fighters out of the Atlantic where they are better used to protect Karelia).

Japan uses its 2 transports to land 4 infantry on Manchuria/French Indo-China.  Japan uses its 3 fighters (the fighter on Philipines can't attack rd. 1 as it's too far) to support troops from Kwangtung/Manchuria to press into Eastern China. This way you kill U.S.'s fighter and troops remaining can either attack India or Manchuria depending on Russia/UK moves going forward.

Conclusion: Don't strike Sikang - it immediately places a weak surviving army at the mercy of UK troops. Instead leave a stronger surviving army drawing from Manchuria/Kwangtung troops into China as this consolidates a more core position (if any of your border countries are attacked you can more easily draw from more countries in subsequent counters).  The biggest key to Japan is to methodically grow by winning battles using big forces to take on smaller allied forces caught off-side with the aide of the maneuverability Japan's 4 fighters and 1 bomber offer.