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 $234.53   13.244%   $265.5912   $265.59   $265.60   $265.60  
  Purchase #2 $122.84   10.491%   $135.7271   $135.73   $135.75   $135.70  
  Purchase #3 $27.69   9.122%   $30.2159   $30.22   $30.20   $30.20  
  Purchase #4 $49.67   13.670%   $56.4599   $56.46   $56.45   $56.50  
  Purchase #5 $127.72   8.419%   $138.4727   $138.47   $138.45   $138.50  
  Purchase #6 $205.34   4.791%   $215.1778   $215.18   $215.20   $215.20  
  Purchase #7 $223.46   5.693%   $236.1816   $236.18   $236.20   $236.20  
  7 purchase totals = $991.25       $1,077.8262   $1,077.83   $1,077.85   $1,077.90