Absolutely not, and I think you should. They may still say no, but it can be a valuable tool in your salary negotiation. I've done this before when switching jobs -- "you want me to start in March, but I have $X in options that vest in July; to switch I would need you to add $X to my signing bonus". Both times they've agreed to do so. Recruiters seem to generally have more leeway with signing bonuses than salaries. (And of course on top of this you should be aggressively negotiating your salary etc.)