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 $20.66   4.138%   $21.5149   $21.51   $21.50   $21.50  
  Purchase #2 $170.39   11.183%   $189.4447   $189.44   $189.45   $189.40  
  Purchase #3 $205.73   10.405%   $227.1362   $227.14   $227.15   $227.10  
  Purchase #4 $6.37   8.403%   $6.9053   $6.91   $6.90   $6.90  
  Purchase #5 $198.33   5.471%   $209.1806   $209.18   $209.20   $209.20  
  Purchase #6 $100.28   10.567%   $110.8766   $110.88   $110.90   $110.90  
  Purchase #7 $91.75   8.182%   $99.257   $99.26   $99.25   $99.30  
  7 purchase totals = $793.51       $864.3153   $864.32   $864.35   $864.30