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 $123.80   4.139%   $128.9241   $128.92   $128.90   $128.90  
  Purchase #2 $46.38   8.826%   $50.4735   $50.47   $50.45   $50.50  
  Purchase #3 $102.33   9.814%   $112.3727   $112.37   $112.35   $112.40  
  Purchase #4 $225.28   9.341%   $246.3234   $246.32   $246.30   $246.30  
  Purchase #5 $82.83   9.503%   $90.7013   $90.70   $90.70   $90.70  
  Purchase #6 $96.60   12.559%   $108.732   $108.73   $108.75   $108.70  
  Purchase #7 $209.97   4.634%   $219.7   $219.70   $219.70   $219.70  
  7 purchase totals = $887.19       $957.2270   $957.21   $957.15   $957.20