You should usually avoid transmission flushes as oil change centres. The flush can damage the seals on your transmission.
After being told 4 times to get a transmission flush at oil change centres, I took my car to a transmission expert and he said it is best to just leave it alone. The only time to worry about your transmission is if you are having trouble shifting gears.
Generally oil change centres make their money off of the "extras" - like transmission flushes, air filters, premium oil, etc. So they try very hard to sell these. Most of them you don't need though, or you can do yourself