It used to be that when we were assigned an en route altitude we held
it, plus or minus a needle width or so -- husbands trained as
engineers are apt to be a little compulsive about things like that.
Well, we had a long discussion with another pilot who had different
advice. He suggested, since we are in a low winged airplane, we hold
assigned less 50 feet or so -- if a high winged airplane, he'd have
recommended assigned plus 50. His obvious intent is to avoid en route
conflicts. It seems like a cheap insurance policy to me, but how real
is the threat? Does anyone have an idea on how often there are
altitude conflicts when on an IFR flight plan?

We have agreed on VFR cross country flights (something that we very
rarely do) we will hold that kind of offset to the altitude rule as a
matter of routine: you might consider doing that too.