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 $27.82   8.104%   $30.0745   $30.07   $30.05   $30.10  
  Purchase #2 $176.30   7.411%   $189.3656   $189.37   $189.35   $189.40  
  Purchase #3 $154.82   12.574%   $174.2871   $174.29   $174.30   $174.30  
  Purchase #4 $90.75   7.745%   $97.7786   $97.78   $97.80   $97.80  
  Purchase #5 $19.77   7.589%   $21.2703   $21.27   $21.25   $21.30  
  Purchase #6 $28.47   8.386%   $30.8575   $30.86   $30.85   $30.90  
  Purchase #7 $44.47   8.672%   $48.3264   $48.33   $48.35   $48.30  
  7 purchase totals = $542.40       $591.9600   $591.97   $591.95   $592.10