Jane it's funny you mention it -as you know I had my transfer yesterday and I am getting my hair done Friday night! I hadn't even considered it being an issue to be honest. At this early stage there's no placenta so no chance of any 'chemicals' that I may/may not absorb getting to the embie (that's just my non-professional view!).
But what it comes down to is whether you would blame a BFN on having gotten your hair done in the TWW. Myself, I'm still having my appt, but you have to be happy with your own decision