DWW-
Not an instructor personally - quite the opposite - but the CFI at my UK-based FTO is adamant that you need Air Law and the medical prior to first solo, and this also accords with my own understanding from the Air Law syllabus itself.
Speaking personally it would seem a bit daft otherwise - you're P1, legal commander of the aeroplane as a solo student, so you should know what you're in danger of transgressing :-)