Excuse me for the spam, but against make it the default behavior I have a simple consideration: what will expect a person that reads the code, that doesn't know Python?
IMHO it expects that the string is *exactly* like it's written. The fact that it will be de-dented it's a bit surprising.
For readability and for not breaking old code, I continue to be in favor of a letter before the multi-string. Maybe `d`, for de-dent, it's more appropriate than `t`, since it does not only trim the string.
But probably there's a better solution than the letter.