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 $17.33   4.326%   $18.0797   $18.08   $18.10   $18.10  
  Purchase #2 $16.63   12.363%   $18.686   $18.69   $18.70   $18.70  
  Purchase #3 $143.86   13.356%   $163.0739   $163.07   $163.05   $163.10  
  Purchase #4 $148.46   8.285%   $160.7599   $160.76   $160.75   $160.80  
  Purchase #5 $63.97   12.413%   $71.9106   $71.91   $71.90   $71.90  
  Purchase #6 $244.48   7.873%   $263.7279   $263.73   $263.75   $263.70  
  Purchase #7 $242.52   10.108%   $267.0339   $267.03   $267.05   $267.00  
  7 purchase totals = $877.25       $963.2719   $963.27   $963.30   $963.30