Ok I'm going to get a little technical here because we are all not the same height and that is going to make a HUGE difference in the height need for our tables.
First let me say that I am 4 feet 11 inch tall, yep I said tall. LOL
Second, I want to say that this pass winter while cutting fire wood I decided to move a log weighting in the neighborhood of 150 lbs. Needless to say it moved me more then I moved it. Long story short. I now have two bulging disk in my neck that can bring me to my knees if I didn't make efforts to make my cutting table more suited to my height.
With that being said, my table was about 27 1/2 " tall. For someone my height you might think it was good enough. I did until the log thing. In order to be able to work on anything without further problem to my neck and back, it had to be raised. What I did was get some heavy duty rollers that are usually used on the large shop tool boxes and mounted them to the bottom of each table leg. This raised the table 3 1/2" so now I have a table that is 32" off the floor. Which makes it about 4" below my waist. At first it took some getting use to but as time passed I found that it has made my work so much easier and I no longer have problems with my neck or back. It also made it easier to clean up since I can now roll the table around when I need to and then lock it in place as well.
I would suggest that everyone look at what they are using and experiment with different heights (using blocks of wood or what ever) so they can fine one that works well for them. Then make the changes because if you don't, later on it will be a problem and in some cases it could make you loss interest in sewing because of the pains from leaning over your table.