I use \mathop{}\!{\mathrm d}} myself. That is a bit convoluted, but it takes care of the spacing in just about every context.. Of course, I put that in a macro

I use \mathop{}\!{\mathrm d}} myself. That is a bit convoluted, but it takes care of the spacing in just about every context.. Of course, I put that in a macro definition so I don’t have to sprinkle it all over my document.