Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $78.83   9.204%   $86.0855   $86.09   $86.10   $86.10  
  Purchase #2 $203.87   11.130%   $226.5607   $226.56   $226.55   $226.60  
  Purchase #3 $102.86   13.547%   $116.7944   $116.79   $116.80   $116.80  
  Purchase #4 $41.92   11.263%   $46.6414   $46.64   $46.65   $46.60  
  Purchase #5 $169.55   9.172%   $185.1011   $185.10   $185.10   $185.10  
  Purchase #6 $40.83   9.624%   $44.7595   $44.76   $44.75   $44.80  
  Purchase #7 $89.50   10.176%   $98.6075   $98.61   $98.60   $98.60  
  7 purchase totals = $727.36       $804.5501   $804.55   $804.55   $804.60