Do you empty out your gas tank at the gas station before filling it up? Hell no! Just the same as you shouldn't empty your bottle before refilling it. If some one is saying they are emptying the bottle before they refill it they are either lying to you or they are stupid. What I mean is they are just filling the bottle and charging you for a whole fill or they don't know they can just 'top it off'. It is a common practice to fill up a half filled bottle. Go to any track when a nitrous vendor is there and ask them to top off your bottle. You will have no problem and you will only pay for what is pumped into your bottle.

Mark