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 $82.66   6.709%   $88.2057   $88.21   $88.20   $88.20  
  Purchase #2 $157.90   4.260%   $164.6265   $164.63   $164.65   $164.60  
  Purchase #3 $233.96   6.360%   $248.8399   $248.84   $248.85   $248.80  
  Purchase #4 $154.40   10.436%   $170.5132   $170.51   $170.50   $170.50  
  Purchase #5 $14.86   9.566%   $16.2815   $16.28   $16.30   $16.30  
  Purchase #6 $199.97   12.271%   $224.5083   $224.51   $224.50   $224.50  
  Purchase #7 $61.49   11.157%   $68.3504   $68.35   $68.35   $68.40  
  7 purchase totals = $905.24       $981.3255   $981.33   $981.35   $981.30