Hello,
Pins first, trust me, it is best.
1, see Custard's comments above, the thicker the stock the harder it becomes, so gang cutting is just doubling the thickness of your stock, doh! The time saving is negligible anyway, it takes longer to cut through double the thickness (more strokes) cutting the waste and paring to the baseline has to be done for each dovetail just the same.
2, marking the tails through the larger gaps in the pin board is vastly easier, especially with London pattern dovetails (single entry) try getting a knife which has a blade thicker than a saw blade into a single saw kerf dovetail. Japanese sawn tails -forget it!
3, there is more chance of a sawing error in the tail sockets, so done first can be corrected when marking the tails. Cut the tails first and any sawing error in the sockets are for keeps.
4 it is easier to mark and see the mark in side grain than end grain. A knife mark in end grain oak, say, needs a lot of pressure to see it. Pins can be marked in pencil, if cut first and then knifed through to the tails.
5, some dovetails cannot be cut tails first, such as secret mitred or double lapped ones, so why not have a single, universal method?
Of course dovetails can and are cut the other way around successfully by many craftsmen, so perhaps you just have to try both and find out for yourself. But I used to cut tails first and after I changed to pins first I won't go back, if you paid me.
Mike.