Please prepare a sum of local currency, because it will be not convenient when you need to buy something in other countries.
There are at least 3 reasons:
1. More convenient
2. Exchange rate will not affect your mood
3. Avoid someone cheat you during exchange money

I made the 30 hour one way trip to Guangzhou from Shenyang BeiZhan in January, 2013 and then 30 hours more to return to Shenyang a week later on the T12/13. First I would recommend buying the the Delux Soft Sleeper because that gives you a bed in a closed compartment with only 3 other people. The other sleepers are not closed and are 3 beds up: a lower a middle and an upper. The Delux Soft sleeper has its own train car and the train personnel control access when necessary. The compartment had a nice heater and kept us warm when it was 15 below zero (C) outside. Personnel give you a hot water thermos you can leave on the little table. You also get nice slippers to wear during the long ride. The bed is acceptably comfortable - I had a lower berth; and the blanket/duvet and pillow were clean - no complaints. You can buy food and drink in the dining car which is the last car and right after the Delux Sleeper Car; or train personnel sell warm dishes from a cart which they push along the corridors quite skillfully. You can also buy food during the train stops where food is sold right at the platform. The corridor also has little fold-down seats you can use to look out the big windows and get a nice change of perspective. The sit-down toilet was OK, never a problem. And the staff were friendly and helpful even though I don't know much Mandarin. Riding the train in China is a nice way to travel, see the countryside and meet different people. The new Shenyang Bei Zhan is also a great train station - very spacious, like Guangzhou East! Just remember that the Delux sleeper car is at the back of the train so try and take the exit from the waiting room that is nearer the back. Your bed is assigned so you don't have to worry about that. When I got on the train in Shenyang, an older couple from Jilin had tickets for separate compartments and asked me to exchange with one of them - I accepted since I still got to keep my lower berth (which I HAD to have). I was then lucky because there was only a lady and her middle school aged son who knew some English. So we had a nice time. I recommend this trip to anyone who likes to travel by train. I'll get pix later.