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 $8.59   13.739%   $9.7702   $9.77   $9.75   $9.80  
  Purchase #2 $138.70   7.550%   $149.1719   $149.17   $149.15   $149.20  
  Purchase #3 $122.27   11.335%   $136.1293   $136.13   $136.15   $136.10  
  Purchase #4 $199.94   7.817%   $215.5693   $215.57   $215.55   $215.60  
  Purchase #5 $19.77   6.155%   $20.9868   $20.99   $21.00   $21.00  
  Purchase #6 $21.64   5.491%   $22.8283   $22.83   $22.85   $22.80  
  Purchase #7 $231.44   12.357%   $260.039   $260.04   $260.05   $260.00  
  7 purchase totals = $742.35       $814.4948   $814.50   $814.50   $814.50