I have just got a new cello bow as my last one died!
When I was trying them out the cello player in the shop was surprised how tight I made the bow.
I was told by my teacher that it should be tight enough to be able to play a note loudly, and with a slight attack at the beginning, without the hairs touching the bow.
The person in the shop was saying no more than an index finger- but this meant the bow wood kept touching the hairs.
Who's right? (as I do not want to ruin my new bow! )

As far as I can tell, both your teacher and the man in the shop! You don't want the bow too tight, as it can distort the wood. But you don't want the wood being scraped by the strings, either. However, the wood can touch the hairs if it isn't scraping against the strings - you should be able to hear this. I'd loosen the bow a bit until your next lesson, as you could potentionally do more damage with the bow too tight than a bit too loose, and then check with your teacher.

It sounds as if the bow hair is a bit too long. We find this varies a lot with atmospheric conditions which means it is a real pain!

Liz