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 $115.49   9.299%   $126.2294   $126.23   $126.25   $126.20  
  Purchase #2 $16.33   12.417%   $18.3577   $18.36   $18.35   $18.40  
  Purchase #3 $171.85   6.488%   $182.9996   $183.00   $183.00   $183.00  
  Purchase #4 $72.93   4.691%   $76.3511   $76.35   $76.35   $76.40  
  Purchase #5 $79.71   11.295%   $88.7132   $88.71   $88.70   $88.70  
  Purchase #6 $242.73   5.756%   $256.7015   $256.70   $256.70   $256.70  
  Purchase #7 $206.53   7.414%   $221.8421   $221.84   $221.85   $221.80  
  7 purchase totals = $905.57       $971.1946   $971.19   $971.20   $971.20