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 $50.37   8.310%   $54.5557   $54.56   $54.55   $54.60  
  Purchase #2 $20.86   7.104%   $22.3419   $22.34   $22.35   $22.30  
  Purchase #3 $125.42   5.357%   $132.1387   $132.14   $132.15   $132.10  
  Purchase #4 $26.67   9.596%   $29.2293   $29.23   $29.25   $29.20  
  Purchase #5 $177.97   13.791%   $202.5138   $202.51   $202.50   $202.50  
  Purchase #6 $44.63   3.384%   $46.1403   $46.14   $46.15   $46.10  
  Purchase #7 $138.26   4.297%   $144.201   $144.20   $144.20   $144.20  
  7 purchase totals = $584.18       $631.1207   $631.12   $631.15   $631.00