Mick, you only need to vent them if it seems they aren't universally hot, or hear a knocking on the pipes. There were a few in the front of the run that I needed to vent. The way I do it is figure out where the first radiator is situated on the beginning run from the furnace, and start there. Many times you can skip the hot ones. There are also issues if the radiator is near the end of the run, and especially long or oversized radiators.

My girlfriend had 2, one going form her son's room to her Dad's room, same pipe, at the end of that run. It took me forever to figure out that combo wasn't workiing. I swapped out the radiator on the end for one half that size, and now it works better.

They sell different size steam vents, for the most part you can use the cheaper universal ones. The next time you're in the big box store, look at the chart they display along the rack there. You can also fine tune things by putting in the vents they recommmend, but in reality you mostly want to concentrate on the beginning and the end of the run with the different vents. And there is a variable vent that can be manipulated with a screwdriver. I would only use this in certain areas where you are having problems, or maybe the room is poorly insulated to begin with.

Above all, watch when screwing and unscrewing not to strip the threads. I use teflon tape on all old fittings. This is good to remember when you hear the hot steam coming like a steam train and need to screw that sucker in quick!