Tearing is better, and if you are washing your fabric when you get it home like you should, then you will have to remove the selvage edge before you wash and they make strings too!
I am old school textile industry and believe me, if you only knew how poorly the fabric is now made even at $12 a yard, you would beg them to rip to make sure you are getting it on grain!
The only exception to this is usually plaids and checks..but I have torn homespun plaid that were 100% on grain before!
Oh and on French sheers you always PULL a thread and then cut....but who uses those any more...
s for pre-washing...it is a MUST in today market place....do you know how many chemicals on that fabric just to get into this country...not to mention other dirt from assorted warehouses and transport boxes...I am by no means a germaphobe, but honestly, those chemicals are causing lots of issues with headaches, eye irritation, skin issues...it is best to be safe....