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 $46.54   8.726%   $50.6011   $50.60   $50.60   $50.60  
  Purchase #2 $30.98   11.413%   $34.5157   $34.52   $34.50   $34.50  
  Purchase #3 $136.96   12.727%   $154.3909   $154.39   $154.40   $154.40  
  Purchase #4 $96.63   11.321%   $107.5695   $107.57   $107.55   $107.60  
  Purchase #5 $62.96   10.749%   $69.7276   $69.73   $69.75   $69.70  
  Purchase #6 $246.35   13.453%   $279.4915   $279.49   $279.50   $279.50  
  Purchase #7 $84.60   4.448%   $88.363   $88.36   $88.35   $88.40  
  7 purchase totals = $705.02       $784.6593   $784.66   $784.65   $784.70