I also round up and trim down. The HST are consistently the size I need and that makes the next piecing step go much more smoothly.
As far as how to do it, here's how I do it. This assumes that you have sewn a straight line when you put the HST together. I also recommend a LOT of starch.
I use the smallest square ruler I have that is bigger than my unfinished square (for a 4.5" square I have a 6.5" ruler). Put the diagonal line of the ruler on the seam with the point at 12 o'clock, and slide the ruler until there is at least a tiny bit of fabric outside the ruler all across both top edges. Trim both top edges, then turn the square around so the untrimmed sides are on the top. Put the diagonal line on your seam, and line up the 4.5" measurement on the bottom two edges and trim. You now have a perfect 4.5" HST with the seam centered.
I have also used Rhonda's method of cutting HST from strip sets (you can find her tutorial in the tutes section). You sew two strips together and use a square ruler to cut the HST out of the middle. I have used this method once and got great results, and I needed 146 HST.
Hope this helps.
Darren