Hopefully Rich or ochster will weigh in; they seem to possess the best expertise. Untill then, I'll relate my experience ( which very well may be controverted)
I'm the "de-facto" amatuer mechanic for several XRs and have done valve adjustment several times. Here's my tips:
Motion Pro makes a great guage, which is bent and narrow and fits nicely into the access port. I've tried standard feeler guages and bent them to fit, but they're too wide and made it difficult for me to get flat and avoid a bad adjustment.
Alignment for TDC is well expressed in the manual. I was impressed how easy Honda made the access and alignment of TDC; just pay particular attention to avoid going the wrong direction, even a little, as it could actuate the auto-compression release causing a bad exhaust valve adjustment. You just need to go all the way around again if the crank slips the wrong direction when aligning the "T" mark.
What isn't clearly expressed in the manual is how "snug" to tighten the adjustment on the feeler guage. I went conservative the first few times and later "snugged" more since the valves were a little noisy. Best I can express it is to avoid so tight that you can feel the spring, but go tight enough that you have to pull the gauge out, as opposed to sliding it out with little to no resistance. Forum wisdom seems to recommend erring the the loose side if you're not sure.
Finally, ochster "educated" me on the possibility (apparently moreso on newer engines) of decreased lash caused by valve seating. Knowing this, I'm re-checking adjustment on the newer bikes more frequently.
Personally, I recommend doing it yourself, since you'll be more inclined to re-adjust more frequently.