I'm a leftie and I cross 'f' and 't' from left to right. I don't remember learning this, it just feels natural to me.
I have been told that I make other letters 'backwards', they don't look reversed but I guess the way I make the letter is backwards to what righties do, kwim?
In school, as a leftie, I was just left to my own devices to adjust so I think I just did what felt 'right' to me.
My thoughts on handwriting are that we use a standard system so that we can communicate and that the 'proper' method helps with clarity, speed and less fatigue.
My backward letters don't slow me down, tire my hand or reduce clarity so I don't see the need to change. If they were a problem I would try to change. If crossing letters right to left is slowing your leftie down or making the letters hard to read it might be worth trying the other way, however if it's not a problem I wouldn't bother, imho.
hth