I use an angle grinder with a flap-disk wheel. I have done the sharpening this way for part of last season, and all of this season. I started with a grinding wheel, but changed to the flap-disk (36 or 60 grit). I like the flap-disk much better.
Your question is an important one. I put the blade in my vise, flat position (blade plane is horizontal). My vise swivels, so I move it into a position that make my passes with the grinder as comfortable as possible. Also, I make sure that I position myself so that the light from the fixture over the workbench reflects well on the sharpened surface (the angle of swivel also is important in this regard).
When ready to put the wheel to the work, I tend to lock my upper body into a fixed position. Any body movement is from the waist down. This helps me to keep the grinder at the same angle as I move it back/forth across the sharpened surface. The reflected light enables me to be certain the wheel is mating across the entire surface, not just the point, or not just the heel. I make sweeps across the entire length, back/forth.
I asked in another thread about angle. I seemed to recall that I read in a thread long ago the ideal angle is 37 degrees. I received no response from my question to affirm this angle. I use notched high-lifts, or full-formed blades (no notches). I checked a few of mine recently, and they were about 35 degrees, as best I could measure.
Anybody have any better information regarding the angle?