Let's take a Congressional election for an open seat as our example. No third party candidates are contesting it so only the Yellow and Purple party candidates are dividing the vote. Let's look at how the rest of Congress will look at the winning candidate based on his/her vote percentage:
50%+1 to 50%+500: Ignore. There'll be a long, nasty recount and by the time the new Congressman is seated the committee assignments will have been handed out. And the recount has a good chance of seating the other guy instead.
50.4% to 51%: Support/oppose re-election. Clearly this winner is vulnerable, so he'll be focused on trying to keep his seat. All votes will be aimed at his district's attitudes. Not available for making deals.
52% to 55%: Work with. This is a solid incumbent who can participate in deal-making as an equal.
55% plus: Respect. This Congressman could be re-elected forever. Don't piss him off, he'll have plenty of chances for payback. He'll have superior leverage in deals.
(75% or more usually means there wasn't a real opponent)
Given those levels an additional vote always has value until the election becomes a total blow-out. If your candidate is leading, vote to elevate him to the next level of clout in Congress. If not, vote to keep the opponent from becoming stronger.
The same logic works for state and local legislatures. Executives also benefit from larger victory margins. A mayor with 58% of the vote can cow the city council into going his way lest he ask his supporters to turn against them.
So vote. Totals matter.