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 $36.30   6.132%   $38.5259   $38.53   $38.55   $38.50  
  Purchase #2 $199.99   8.429%   $216.8472   $216.85   $216.85   $216.80  
  Purchase #3 $104.63   12.375%   $117.578   $117.58   $117.60   $117.60  
  Purchase #4 $87.38   12.799%   $98.5638   $98.56   $98.55   $98.60  
  Purchase #5 $239.25   5.200%   $251.691   $251.69   $251.70   $251.70  
  Purchase #6 $71.40   7.701%   $76.8985   $76.90   $76.90   $76.90  
  Purchase #7 $166.54   6.802%   $177.8681   $177.87   $177.85   $177.90  
  7 purchase totals = $905.49       $977.9725   $977.98   $978.00   $978.00