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 $107.48   11.224%   $119.5436   $119.54   $119.55   $119.50  
  Purchase #2 $48.99   6.419%   $52.1347   $52.13   $52.15   $52.10  
  Purchase #3 $118.72   13.658%   $134.9348   $134.93   $134.95   $134.90  
  Purchase #4 $59.90   9.229%   $65.4282   $65.43   $65.45   $65.40  
  Purchase #5 $150.95   10.315%   $166.5205   $166.52   $166.50   $166.50  
  Purchase #6 $235.67   9.199%   $257.3493   $257.35   $257.35   $257.30  
  Purchase #7 $173.49   5.746%   $183.4587   $183.46   $183.45   $183.50  
  7 purchase totals = $895.20       $979.3698   $979.36   $979.40   $979.20