I did worry a bit about this to start with. In fact, I pm'd a few people back to explain that I was going to put the placemark a little way away from their exact location, but never got any replies.
Because of this, and with the huge amount of new entries (I get more in a day than I did in a Month previously) and the time it takes, then the placemark will generally be added exactly where you put your postcode.
So, if you're worried, please only supply the first half.
It does vary a bit, the details people supply. So far, the vaguest location is "M25" and the most detailed inclded instructions included a few lines on how to pinpoint the exact house
While I'm waffling, I used to pm people back when I added them (seems only polite) but due to the number now, and having to wait 60secs before sending each pm, I no longer do that.
If you do spot any mistakes in your entry, please just pm me and I'll edit it.