I know this will sound ignorant but when tearing fabric instead of cutting, do you tear from selvedge edge to selvedge edge or the other way ?

That's not dumb!!
I've done both, actually, but usually the selvedge edge

ditto.. :thumbup:

It can be done either way, it just depends on what you need.

When tearing from selvedge edge to selvedge edge, you are tearing on the crossgrain, which is harder to do than straight-grain tearing. It also produces a more distorted edge, and more microscopic damage farther in along the edge (up to 2 inches).

A lot of quilters tear only to make long strips for borders, in which case they tear on the straight-grain (parallel to a selvedge). Everything else is cut rather than torn.

I guess the big question is, why do you want to tear your fabric?

Just to make a piece of fabric more managable

In that case, I would cut crossgrain rather than tear. It leaves a better edge, with no damage to the fabric.

which ever way you need to (make your cuts) if you want to cut long borders- and plan to cut length wise- well- tear length wise- if you always cut salvage to salvage---tear it that way- the tearing is to have a straight (on grain) edge- the grain runs both ways

Ditto here too

The quality of the fabric can affect the distortion when tearing.