Replacing a tyre

[raleighnut]

I've never heard the 90⁰ before and have always done 180⁰.

That makes it more difficult to get the last bit on as you're 'fighting' the valve (it won't allow the bead to drop into the rim centre well) at least with the 1/4 turn (90 degrees) the part of the tyre opposite is able to drop into the centre more easily