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 $64.99   13.630%   $73.8481   $73.85   $73.85   $73.80  
  Purchase #2 $223.32   10.580%   $246.9473   $246.95   $246.95   $246.90  
  Purchase #3 $153.25   4.470%   $160.1003   $160.10   $160.10   $160.10  
  Purchase #4 $150.30   10.189%   $165.6141   $165.61   $165.60   $165.60  
  Purchase #5 $43.49   12.550%   $48.948   $48.95   $48.95   $48.90  
  Purchase #6 $191.49   3.540%   $198.2687   $198.27   $198.25   $198.30  
  Purchase #7 $87.95   10.155%   $96.8813   $96.88   $96.90   $96.90  
  7 purchase totals = $914.79       $990.6078   $990.61   $990.60   $990.50