In general, the cruise lines are good about fare reductions prior to final payment, nasty about them afterwards (the close in "flash" or "happy hour" fares are restricted to new bookings only). However, that doesn't mean that all future promotions such as onboard credits, pre-paid gratuities or other onboard perks offered between now and then would be honored by the cruise line after you've already booked.
It would be helpful to know what line you're looking at so we could give more specific guidance as to their policies and tendencies.
By the way, be sure to ask your agent about their fare reduction policies, not just if they honor reductions, but if there are any service fees associated. Many discount agencies charge a change fee every time you alter your booking