I find it depends on whether you are doing a 3, 4, or 5 major build and how high you are taking your 1-2-3 attributes.
On my three major builds, I've decided to quad enhance my first two (e.g. punting/strength) and I'm triple training them until punting is 99.
http://goallineblitz.com/game/player.pl?player_id=4429599He's currently sitting at 98.73 punting and 84.72 strength.
Once punting is finishing training, I'll switch it out as the free training slot for the fourth attribute and will probably buy the fifth attribute multi so that I can quad train going forward.
With this one:
http://goallineblitz.com/game/player.pl?player_id=4429481I stopped training at 95 on my primary because on a STOP it is better to have all five of your top attributes as high as possible.
I 3x enhanced all five of my first five attributes for the same reason.