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 $74.35   8.626%   $80.7634   $80.76   $80.75   $80.80  
  Purchase #2 $111.71   12.217%   $125.3576   $125.36   $125.35   $125.40  
  Purchase #3 $71.36   4.299%   $74.4278   $74.43   $74.45   $74.40  
  Purchase #4 $67.57   3.211%   $69.7397   $69.74   $69.75   $69.70  
  Purchase #5 $208.55   6.421%   $221.941   $221.94   $221.95   $221.90  
  Purchase #6 $69.62   12.487%   $78.3134   $78.31   $78.30   $78.30  
  Purchase #7 $231.63   12.107%   $259.6734   $259.67   $259.65   $259.70  
  7 purchase totals = $834.79       $910.2163   $910.21   $910.20   $910.20