The alarms might not go off until the nose wheel crosses it, but in GA land I've been taught that no part of the aircraft should cross the line. Especially important when doing power checks into wind, when I might be sitting parallel to the line the wing shouldn't cross it.
Perhaps should think about the reason for the position of them. They are set to allow enough clearance for a certain sized aircraft (I know that Prestwick had to change their hold lines when preparing to accept a new type - possibly 747-8F) to pass safely and also to protect the ILS system for CAT landings.
There is going to be a small amount of contingency but not alot as space is always at a premium. The clearance will be measured from the centre line marker to the hold point.
If your nose is over the line, you are going to be vulnerable to getting it caught by someone else, and potentially you could even interfere with avionics of landing aircraft. Plus you wont be able to see whether the stop bars are illuminated (if the hold is so equipped).
From a CRM prospective - better to do the callout in my opinion. The captain may not have realised he was still inching forward.