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 $19.96   4.568%   $20.8718   $20.87   $20.85   $20.90  
  Purchase #2 $73.28   3.356%   $75.7393   $75.74   $75.75   $75.70  
  Purchase #3 $166.54   6.543%   $177.4367   $177.44   $177.45   $177.40  
  Purchase #4 $109.30   9.298%   $119.4627   $119.46   $119.45   $119.50  
  Purchase #5 $161.81   5.351%   $170.4685   $170.47   $170.45   $170.50  
  Purchase #6 $81.60   8.307%   $88.3785   $88.38   $88.40   $88.40  
  Purchase #7 $143.33   8.231%   $155.1275   $155.13   $155.15   $155.10  
  7 purchase totals = $755.82       $807.4850   $807.49   $807.50   $807.50