Hi

My wife owns a show bred labrador who is 4 months old. She is wondering when she should start trimming the tail as it has a squarly bit on the end

I would get them used to it as soon as possible, although I dont have to trim tails on Labs, just springers.  The sooner you start getting them accustomed to it, the sooner you will have no problems with them fussing. 

I would check with your breeder/mentor to see if trimming in this area is usual, as I would expect a natural appearance to be the norm, with no trimming except nails.

Yes trimming the tail end is completely normal ;)  take the tip of tail in your hand and using your thumb as a guide, trim to a nice rounded shape so there is no curl or untidiness..............I usually do my girls tail whilst she is eating her dinner as she's too busy to disagree about it :D

p.s. I think it's done as it gives the appearance of a thicker rounder otter like tail ;) and just looks neater if you were going to show, however, it isn't neccessary if you've no intentions of entering a show ring ;)