nlapiSetLineItemValue on Transaction Line Item Amount

This doesn't seem to work with the amount field...

nlapiSetLineItemValue('item','amount',n,100);

It works fine with nlapiSetCurrentLineItem though.  My objective is to walk through opportunity lines and update amounts.  I'd like to avoid nlapiSetCurrentLineItem because it requires selecting each line first, and traversing the lines this way seems to have sync issues.  It also yields annoying messages for each line... "The line total amount is not equal to the item price times the quantity.  Is this correct?"

Is there any reason why I can't change the amount field with nlapiSetLineItemValue?

Matt