Losing steps...

I've recently discovered my machine is losing steps, as steppers do,
and so I'm trying to figure out if some (small) amount of missed
steps is "normal." The resolution on my machine is quite small
(.000098"), and I'm wondering if it's just too much to ask to expect
it not to lose any steps at all. I can usually re-home the machine
after a fairly long program and get less than .0003" "homing error",
(measured with the homing switches, which are purported to be
repeatable to .000039" when new). I'm really not getting wrapped
around the axle with meaninglessly small numbers, but I would like to
know if a properly working stepper system can be expected to miss no
steps? At what point do I declare a problem? 3 missed steps or 10
missed steps?

Also, I was reading Mariss' white papers on stepper vs. servo, and I
need some clarification on one of the disadvantages of stepper
systems: 3) Low accuracy. 1:200 at full load, 1:2000 at light
loads. Can someone explain exactly what the above statement means.

TIA,

MP