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 $64.86   13.677%   $73.7309   $73.73   $73.75   $73.70  
  Purchase #2 $79.99   4.602%   $83.6711   $83.67   $83.65   $83.70  
  Purchase #3 $83.47   9.454%   $91.3613   $91.36   $91.35   $91.40  
  Purchase #4 $249.46   3.183%   $257.4003   $257.40   $257.40   $257.40  
  Purchase #5 $12.43   12.128%   $13.9375   $13.94   $13.95   $13.90  
  Purchase #6 $233.48   9.220%   $255.0069   $255.01   $255.00   $255.00  
  Purchase #7 $28.44   12.296%   $31.937   $31.94   $31.95   $31.90  
  7 purchase totals = $752.13       $807.0450   $807.05   $807.05   $807.00