Sharon -
I do unhooped as much as possible because it is flexible and easier on my hands. I hope you saw my 'hooping blog' that I posted on 6/6 in The Avid Embroiderer.
I have used a standard ruler to assist me with centering. It worked sort of OK. Then, I recently got a "Placement Kit" and am delighted to see that when I place my fabric, and the grain is just a little off, I can fix that with the kit and my machine. Not all machines can do that, but this is really great if you have it.
I think it boils down to determining where YOU decide where the center is and make adjustments with or without tools. I personally cannot do a good job without the tools. I cannot draw a straight line with a ruler and 2 good looking engineers.....
Pat
I hope you'll visit The Avid Embroiderer my blog for some tips and tricks from someone who enjoys sharing her discoveries.