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 $23.35   13.409%   $26.481   $26.48   $26.50   $26.50  
  Purchase #2 $38.29   10.737%   $42.4012   $42.40   $42.40   $42.40  
  Purchase #3 $242.75   8.541%   $263.4833   $263.48   $263.50   $263.50  
  Purchase #4 $9.94   7.623%   $10.6977   $10.70   $10.70   $10.70  
  Purchase #5 $127.59   10.633%   $141.1566   $141.16   $141.15   $141.20  
  Purchase #6 $145.85   3.365%   $150.7579   $150.76   $150.75   $150.80  
  Purchase #7 $27.92   3.497%   $28.8964   $28.90   $28.90   $28.90  
  7 purchase totals = $615.69       $663.8741   $663.88   $663.90   $664.00