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 $24.91   6.253%   $26.4676   $26.47   $26.45   $26.50  
  Purchase #2 $195.78   13.868%   $222.9308   $222.93   $222.95   $222.90  
  Purchase #3 $35.29   7.811%   $38.0465   $38.05   $38.05   $38.00  
  Purchase #4 $148.54   10.146%   $163.6109   $163.61   $163.60   $163.60  
  Purchase #5 $17.57   8.769%   $19.1107   $19.11   $19.10   $19.10  
  Purchase #6 $25.57   7.217%   $27.4154   $27.42   $27.40   $27.40  
  Purchase #7 $97.69   13.786%   $111.1575   $111.16   $111.15   $111.20  
  7 purchase totals = $545.35       $608.7394   $608.75   $608.70   $608.70