+ 1 for rollers - they can be very useful. The only downside is if your floor is not even, when they can be a real pain to reposition from one spot to another. Height wise, they should be exactly the same as that of the machine bed, for which the are an extension. I tend to set mine up with a long straight edge off the infeed and outfeed of the particular machine I am using.
They do, however, need a very positive locking mechanism, otherwise they can collapse under the weight of the wood. They also need to be stable at their base, or one is forever having to pick them up should you accidentally knock them with the piece of wood you are machining.
I also use a couple of roller tables that I keep permanently positioned either side of my Radial Arm saw. These are a godsend for positioning large, heavy lengths of wood. But, as they are tucked against one of the walls, they are not prone to getting in the way, as they might on my other machines.