I hope they're going to offer pro rata on seat volumes between multiples of 20. If I have just 25 users it's $10,000 (assuming their pricing widget is completely accurate).
(the point of block pricing is to extract some extra revenue from people in your situation. as a bonus, you get a slight user-license runway so you can add people without paying more if you are in the middle of a license block. also, some companies vastly overestimate their users and will order much more capacity than needed. free money is the best money.)
Spending money you don't have to is always a bad idea, so while the unused license space may not make a huge dent on the bottom line, a company that pays attention to those things will likely outperform one that doesn't.
Additionally, you usually financially plan for annual spend so you would budget for the total number of seats needed by the end of the financial year.