I think there are two big benefits of donating monthly and automatically as opposed to on Giving Tuesday:
1) you can't forget to do it.
2) you don't see a big amount pile up during the year and then on Giving Tuesday think "am I really going to give that much away??"
Of course if you know you won't fall prey to either of these scenarios then it makes more sense to do as you describe.
I think there are two big benefits of donating monthly and automatically as opposed to on Giving Tuesday:
1) you can't forget to do it.
2) you don't see a big amount pile up during the year and then on Giving Tuesday think "am I really going to give that much away??"
Of course if you know you won't fall prey to either of these scenarios then it makes more sense to do as you describe.