Doug Winterburn wrote in news:5511f77a$0$46442
:

It really does make a difference when cutting both sides. The same
rules apply as when cutting one side as the cutting action of the bit
will tend to keep the router against the guide.

How so? If the cutting edge on the right side of the groove
is pulling towards the template, then the cutting edge on
the left side of the groove is pushing away. They cancel
each other out.

Other than when starting a groove with a non-plunge router
it shouldn't make a difference (when starting only one
side of the bit cuts, of course).

John