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 $21.71   13.487%   $24.638   $24.64   $24.65   $24.60  
  Purchase #2 $111.38   8.529%   $120.8796   $120.88   $120.90   $120.90  
  Purchase #3 $73.81   3.775%   $76.5963   $76.60   $76.60   $76.60  
  Purchase #4 $45.76   10.769%   $50.6879   $50.69   $50.70   $50.70  
  Purchase #5 $217.69   6.204%   $231.1955   $231.20   $231.20   $231.20  
  Purchase #6 $193.94   4.676%   $203.0086   $203.01   $203.00   $203.00  
  Purchase #7 $169.31   6.436%   $180.2068   $180.21   $180.20   $180.20  
  7 purchase totals = $833.60       $887.2127   $887.23   $887.25   $887.20