Go with the lowest setting on the charger. I've always used 2 amp but technically trickle charging should be less than that.
I recommend one of those high frequency chargers that are becoming popular. You can now buy them at places like Menards or Walmart. I bought mine from Menards for $20 bucks on sale.
It's all electronic and has push buttons for settings and led indicators (along with audible beep) for each function, even has it's own internal cooling fan. When the battery if fully charged the "full" led lights up and it automatically goes into maintenance mode. It will never overcharge the battery. For larger car batteries (or if you need your bike battery charged in a hurry) you can select 2, 4 or 6 amps and it will autmatically drop to the next level down until it's in maintenance mode.
You have to be careful with a regular charger and the cheap trickle chargers. If you forget to disconnect them, they'll overcharge the battery and boil out all the water destroying your battery.