> If I pay relocation expenses, I require receipts
This might be a deal breaker for me if I was interviewing with someone. It's not that I care about the accountability part, it's more that a ton of relocation costs are unforeseen incidentals, or are private expenses (like first month's rent ... it's not your business what I choose to pay in rent), that should be covered by just paying cash to the new hire. Making them track down receipts for all of it might work out to be more costly, in terms of time lost, than just bearing the taxes, and certainly might involve requiring knowledge of expenses the employee may rightfully prefer to keep private.
Usually, I expect that the company will handle almost all aspects of relocation on their own (e.g. buying plane tickets and hotel rental for housing visit, calling up and hiring the moving company to pack and ship belongings). The part that would be paid as cash would be the cost of rental fees, deposit, and first month's rent, as well as some small amount for incidentals, which should be grossed up.
On the other stuff I guess we just disagree. I don't view sign-on bonus or severance pay as wage pay, even if they are pegged to wage level. For the other stuff, I agree, it's just part of regular income and the employee should bear the tax. But for sign on bonuses and severance, I think it's fair for the employer to bear the tax in light of the purposes of those payments.
Sign on bonuses are very different in this discussion though, since not everyone provides them and the amounts can vary a lot and for low amounts grossing up can matter significantly, while for larger amounts, employees are usually happy to bear the tax cost themselves. I'm not so worried about what an employer's policy is for these.
It would matter to me to negotiate a specific value paid for severance, though, rather than "6 months' salary" which will then have taxes taken out. If an employer was put off by this, it could be a problem, depending on all the other aspects of the job. One way to avoid this is that if secretly what I want is 6 months severance grossed up, then I'll try to negotiate 9 months severance, without mentioning the grossing up part. It's less off-putting to the counterparty, even if it would be nicer if I could just speak plainly.
I'm just speaking from my experience as a candidate in previous roles. I'm sure everyone has different experiences.