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 $199.75   12.462%   $224.6428   $224.64   $224.65   $224.60  
  Purchase #2 $111.50   13.584%   $126.6462   $126.65   $126.65   $126.60  
  Purchase #3 $226.25   13.777%   $257.4205   $257.42   $257.40   $257.40  
  Purchase #4 $106.41   9.284%   $116.2891   $116.29   $116.30   $116.30  
  Purchase #5 $76.29   13.251%   $86.3992   $86.40   $86.40   $86.40  
  Purchase #6 $171.74   4.717%   $179.841   $179.84   $179.85   $179.80  
  Purchase #7 $27.82   5.143%   $29.2508   $29.25   $29.25   $29.30  
  7 purchase totals = $919.76       $1,020.4896   $1,020.49   $1,020.50   $1,020.40