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 $242.73   9.701%   $266.2772   $266.28   $266.30   $266.30  
  Purchase #2 $148.94   12.547%   $167.6275   $167.63   $167.65   $167.60  
  Purchase #3 $17.42   7.492%   $18.7251   $18.73   $18.75   $18.70  
  Purchase #4 $154.43   7.767%   $166.4246   $166.42   $166.40   $166.40  
  Purchase #5 $88.30   6.411%   $93.9609   $93.96   $93.95   $94.00  
  Purchase #6 $16.62   7.737%   $17.9059   $17.91   $17.90   $17.90  
  Purchase #7 $30.33   11.798%   $33.9083   $33.91   $33.90   $33.90  
  7 purchase totals = $698.77       $764.8295   $764.84   $764.85   $764.80