Cake tasting

Simple - you tell her you don't like any of them and you will not be using her. You may be wording it in different ways but one of the worst things about being an independent supplier is being led to believe that someone is interested when they are not. Its much better for the supplier if you just tell them.

Agreed, different people have such different tastes when it comes to flavours and textures so just be honest. If they are not for you someone else will undoubtedly love them and what they hate you may love.

I'd just use the classic "we've got a couple of other tasting sessions lined up with other suppliers this week so will get back to you next week once these are all done". Then you can let her down easily via email rather than in person if needs be.

Is it preferable to taste the cakes from the different cake suppliers, before making a decision on which cake supplier to go with?

Also, is it recommended to see them first as well, before making a decision?

Some cake suppliers have tasting boxes! You can ask if they do tasting boxes instead?