I can understand why you might pull the shirt right down to the base of your penis to make sure all your cum ends up on the shirt as an easy way of cleaning up - you just take off the shirt and put it to be washed. Even so, I wouldn't do that with a shirt that I still wore regularly. Cum does seem to wash out fine, in my experience, but I do hear reports from people saying they have had lasting stains.
And I can see why you would take the shirt off before cumming, maybe even before starting, to make sure none of your cum lands on the shirt and then wipe up with something else, especially if you want to wear the shirt again before it gets washed or you're just be cautious about stains.
But getting some of your cum on it seems like the worst of both worlds. You now have to wash that shirt before you can wear it again and you still need to wipe up the rest of the cum, though you could just use the shirt for that.