Most people aren't comfortable explaining their needs to their bosses. It never hurts to ask. Any half-decent boss is a reasonable person who understands reasonable requests.
A better chair, desk, monitor, or keyboard/mouse costs WAY less than the time lost by bad equipment.
For example: Let's say a chair costs $500. Let's say that a developer's time might cost a company $100/hr (after benefits and taxes). It would only take 5 hours of lost productivity to pay for that chair.
Your boss is never going to know you want that chair or that it would save him productivity unless you ask for it.