Yes A/C does use a little more fuel, and yes older vehicles used even more.. Compressors now are pretty efficient and it is not that noticable..
Not running the A/C regularly can risk the Pipe seals drying out and leaking or at worse, causing the compressor to be starved of oil and therefore running the risk of it seizing. So it should be run at minimum once a week for 20mins or so.. Not running the A/C also allows bacteria growth on the Evaporator, that is why you get that "smelly Sock" odour if it hasnt been used for a while.
Remember also that A/C is not only useful during the hot months.. It acts as a dehumidifier, and that is why in winter when the A/C is switched on, the inside of the windscreen clears of condensation/moisture very quickly..
In reality you should run it all the time.. 1: to protect the working parts. 2: to keep you cool and moisture free..