Well....
According to Airbus, we do a full alignment the first flight of the day using airport coordinates (after verification by looking at a chart). Then, throughout the day, we do fast alignments ONLY if residual ground speed is greater than 5 knots.
The exception is: When flying in areas where updates are not available (for example, over water) AND you are NOT GPS equipped, THEN you do a full alignment with gate coordinates, if available (again, verify, verify, verify).
It is not necessary, nor desirable, to do fast alignments after each landing (assuming the IRUs are performing satisfactorily)...regardless if the aircraft is or is not GPS equipped.
Let's assume you're flying a non-GPS-equipped aircraft. Now, you do the full alignment in the morning (first flight of the day). So, as per SOP, you use airport coordinates, which are probably just a slight bit off from your actual gate position. As the FMGCs get updates from ground-based stations, the FMGC position becomes 'refined'....that is, your FMGC position is probably fairly accurate by the time you make your first landing of the day.
So, now, contrary to Airbus SOP (assuming the residual ground speed is five knots or less.....which it probably is), well-intentioned pilots do fast alignments, again, using airport coordinates. Chances are, the new FMGC position (after this fast alignment) is more 'off' from actual position than before the fast alignment. So, this really isn't a big deal, since, with your subsequent takeoff, you get a runway update AND, after takeoff, the ground-based nav aids will, again, update and refine the FMGC position...just like before...just like it always does. So, what did you accomplish by doing the fast alignment? Nothing!!!! (Maybe you zero'd a residual 2 knot ground speed...big deal!)
The Airbus philosophy is: leave it alone as long as it's working properly. Again, the exception is for non-GPS aircraft that are about to fly a leg over an area where the gound-based position updates will not occur. THEN, you are to do a full alignment WITH gate coordinates (if available).
I fly with F/Os who think they're doing 'above and beyond the call of duty' by doing fast alignments each leg. Then, I show them, the FMCG position was closer to the actual aircraft position BEFORE the fast alignment...they made things worse...not better. (Again, this really is no big deal, since the thing updates with TOGA/FLEX...using runway coordinates...anyway.)
Every time you do any kind of alignment, you need to verify what's going into that FMGC. And, every time you do any kind of alignment, you're risking making an error.
Now, having said all of this...IF you follow Airbus SOP and verify the position of the aircraft vis-a'-vis the FMGC position (after any kind of alignment), then it's all only an issue for discussion anyway. Follow the SOPs and you can't go wrong....
Fly safe,
PantLoad