Most market participants expect the Fed to cut its $US85 billion ($NZ103 billion) in monthly bond purchases, otherwise known as quantitative easing, by $US10 billion to $US15 billion, although some believe the move may come later this year.
Craigs Investment Partners broker Chris Timms said the central bank's stimulus was credited with helping the Standard & Poor's 500 index's 150% rebound from its March 2009 low.
The Dow Jones Industrial Average was a only a few points shy from its August 2 record and the technology rich Nasdaq was at an all-time high.
The NZX-50 has this week reached all-time highs, largely due to expectations around the Fed's actions.
Mr Timms said anything less than a $US20 billion a month in tapering would be seen as positive by the markets.
The markets had been supported by the artificial stimulus of the Fed buying bonds. If it started tapering its support, it would indicate the US economy was growing, companies were growing profits and the economy was starting to independently support itself.
''It is an interesting balance for the Fed. It wants economic growth, it wants to support the economy and it doesn't want to turn the money tap off too much.''
Mr Timms warned against a hasty reaction as there were so many moving parts in play.
As well as the tapering, the Fed might chose to alter its threshold for tightening, perhaps by lowering the trigger level on unemployment from the current 6.5%.
It would also publish its first economic forecasts for 2016 and the stronger the picture, the harder it would be to convince markets any future rise in interest rates would only be slow and measured.
''The Fed has already had trouble convincing the market it intends to keep rates near zero out to 2015 - no matter how much the economy improves.''
The decision and economic projections are out at 6am today while Fed Chairman Ben Bernanke starts his press conference 30 minutes later. Often, markets can react violently to the former, then completely reverse course depending on what Mr Bernanke says.
Mr Timms expects Mr Bernanke's speech to be ''dovish'' in tone, indicating the Fed is unlikely to take aggressive actions.
