My contract affords me 500MB download per month. Currently I leave the phone disconnected unless I want to use the net, because I don't know how much data my phone will transfer when it's just idling.

I have Skype and Google Talk set up, and I assume a small amount of data will be used to maintain the availability statuses of me and my contacts.

Do I need to bother with this, or would the phone at idle use so little bandwidth that there's no point in not leaving it connected all the time?

My newly-acquired Nokia N900 is my first internet phone, so I'm new at this.