Agree with you in principle (your previous deal with company A is absolutely none of company B's business) but I would like to challenge you on one point:
> I'm placed at a disadvantage in negotiations if I name a number first.
... This is unlikely to be true. Generally, in price negotiations it is advantageous to name a number first. This is known as anchoring and basically the theory is that the end result is better for you if the company has to wheedle you down from 170k than if they anchor the price at 100k and you have to argue them up from there.
Plus, it saves a lot of wasted negotiation time on both sides if your price is just way too high for the company and this is clear from the beginning.
(edit: I'm talking about expected salary in general, not disclosing your current salary level)
> I'm placed at a disadvantage in negotiations if I name a number first.
... This is unlikely to be true. Generally, in price negotiations it is advantageous to name a number first. This is known as anchoring and basically the theory is that the end result is better for you if the company has to wheedle you down from 170k than if they anchor the price at 100k and you have to argue them up from there.
Plus, it saves a lot of wasted negotiation time on both sides if your price is just way too high for the company and this is clear from the beginning.
(edit: I'm talking about expected salary in general, not disclosing your current salary level)