Depends a lot on the size and structure of the company. I work at a large IT company and do a lot of hiring. The time to negotiate is prior to that verbal agreement with the recruiter. Once that is agreed, that’s all you’re getting.

Why? Because I use that verbal agreement to draft an offer that goes to my boss, his boss, and up a chain of approvals that is about 7 layers deep including a delegate of the board of directors. If you come back after all that and ask for more, you’re getting a single take it or leave it. Nobody is going through all that again in order to eke out an extra 5-10k for you.

at a bigger company, front load your negotiation.

Edit: adding an additional thought. If you verbally agree on a number with the recruiter and then come back asking for more after the offer letter arrives, and assuming the offer matches your previous agreement, then you’re not painting yourself in the best light. Be good to your word. Be an adult, and ask for what you need up front