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 $242.67   6.738%   $259.0211   $259.02   $259.00   $259.00  
  Purchase #2 $215.83   8.271%   $233.6813   $233.68   $233.70   $233.70  
  Purchase #3 $126.52   4.290%   $131.9477   $131.95   $131.95   $131.90  
  Purchase #4 $148.88   11.802%   $166.4508   $166.45   $166.45   $166.50  
  Purchase #5 $163.66   9.196%   $178.7102   $178.71   $178.70   $178.70  
  Purchase #6 $123.82   3.407%   $128.0385   $128.04   $128.05   $128.00  
  Purchase #7 $176.41   6.253%   $187.4409   $187.44   $187.45   $187.40  
  7 purchase totals = $1,197.79       $1,285.2905   $1,285.29   $1,285.30   $1,285.20