We tend to take the SW corner and call it N1000.00' E1000.00' unless we have actual coords from a survey. That or we take a solid, reliable, stationary, static, (can't emphasize the importance of not picking a point that can be destroyed or moved easily which is a newbie lesson, I know, but I'm a site plan newbie) and call it N1000',+E1000'
That was we always deal with positive numbers, even if, by chance we have to go back to the left of below the datum point (assuming north is up the page)
We don't always pick a column line or whatever, like that... unless we have -no- site data, in which case, we'll take column line intersection A1 and call it N1000,E1000.
It's worked out quite well for us so far in projects both large (large for us) and small.