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 $21.90   4.525%   $22.891   $22.89   $22.90   $22.90  
  Purchase #2 $138.40   10.603%   $153.0746   $153.07   $153.05   $153.10  
  Purchase #3 $5.54   10.784%   $6.1374   $6.14   $6.15   $6.10  
  Purchase #4 $206.49   8.603%   $224.2543   $224.25   $224.25   $224.30  
  Purchase #5 $227.43   11.572%   $253.7482   $253.75   $253.75   $253.70  
  Purchase #6 $201.69   12.620%   $227.1433   $227.14   $227.15   $227.10  
  Purchase #7 $75.72   4.317%   $78.9888   $78.99   $79.00   $79.00  
  7 purchase totals = $877.17       $966.2376   $966.23   $966.25   $966.20