Hi Rose,
I used to worry a lot about currency before long haul tours but I've found that the TM will always organise (or at least be willing to organise) somewhere for the group to change money - either at a bureau in the airport, the hotel, bank or elsewhere. Some currencies you can get before you go, others only when you are in the country. A tour manager recently told me never to change money beforehand, but to get it in the country. I think in SE Asia, it would be easiest to change US dollars, but in most other countries it is just as handy to bring sterling or (even more so) euro. Hope this helps!
All the best,
Bob