Leroux, I think most people will tell you to grind based on the "better safe than sorry" approach. In which, thinking about the time, money, and effort in stripping a failed epoxy job and reapplying it, who can blame them. When I did my Epoxy Coat floor this summer, I had first muratic acid washed and rinsed it, then attempted sanding it with a 25 grit rotary sander, then eventually ground it with an Edco diamond stone grinder (I think more paranoia than anything).
Here's what I learned. I honestly feel the muratic acid wash and rinse did the best job. The sander and grinder did comparable jobs, but I had to change sanding disks about every 50 square feet, which can add up. The grinder did a decent job at knocking down any high spots left from a power trowler, but unfortunately, left a nearly polished surface on the remains of them. Put your extra effort into filling in any low spots if you're really worried about it (the epoxy will sit level at first, but will slowly flow for the first hour or two, cracks WILL stand out a little more), otherwise, use the flakes and forget about it).
When you put the epoxy down and start to squeegie it around, it'll look really thin in spots, don't worry, the roller does a really good job at leveling it, just keep with the instructions. When I did mine, cutting in the edges, I had about 30-35 minutes of working time before it started getting thick, so don't mix too much at a time until you get a feel for it.
Here's how mine turned out:
http://www.garagejournal.com/forum/showthread.php?t=19399&highlight=two+color+epoxy+floor