Not true.
US law states: “A signature may be written by hand, printed, stamped, typewritten, engraved, photographed, or cut from one instrument and attached to another, and a signature lithographed on an instrument by a party is sufficient for the purpose of signing it, it being immaterial with what kind of instrument a signature is made. ... whatever mark, symbol, or device one may choose to employ as a representative of himself is sufficient.
I stand corrected; every legal document I have ever seen had a signature (in cursive) upon it, so I assumed it was a legal requirement.