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 $242.53   6.113%   $257.3559   $257.36   $257.35   $257.40  
  Purchase #2 $178.69   5.178%   $187.9426   $187.94   $187.95   $187.90  
  Purchase #3 $112.42   11.115%   $124.9155   $124.92   $124.90   $124.90  
  Purchase #4 $24.35   9.793%   $26.7346   $26.73   $26.75   $26.70  
  Purchase #5 $95.81   10.809%   $106.1661   $106.17   $106.15   $106.20  
  Purchase #6 $29.26   5.438%   $30.8512   $30.85   $30.85   $30.90  
  Purchase #7 $29.55   5.872%   $31.2852   $31.29   $31.30   $31.30  
  7 purchase totals = $712.61       $765.2511   $765.26   $765.25   $765.30