I would take a reading at pitching time... sometimes I'll then use that to work out my dilution addition if I've prepared an over-gravity wort. Last time I didn't bother with a true OG after dilution. Oh well.
I used to just float the sanitized hydro in the fermenter for the OG, but its so damn hard to get rid of the foam to make the reading if you did a good job aerating!
Then I wait until after high krausen, in fact, I wait until the yeast enters conditioning and the airlock stops bubbling for the first sample. I'll generally flush the first half sample to clear the trub out of the taps etc. Then take the real sample... give it a taste... if its awesome I'll drink more of it... if its really awesome might stick it in a glass in the fridge to share with others
SWMBO normally isn't interested in it if its remotely bitter, but if its a sweet one... then I can sometimes get her to sample it
I'll then generally sample every few days until the brew finishes... then probably leave it till the next weekend anyway... gets me better beer.
I actually use about 125ml for a sample becaus I got a wider sample jar, makes reading it easier.
I plan to keg 19L, so I just make up 20L into the fermenter, lose about 500ml to samples and a lil bit for trub, and get 19L in the end... I'd rather come short, than have to fill a few bottles in order not to "waste" beer.