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 $50.53   9.643%   $55.4026   $55.40   $55.40   $55.40  
  Purchase #2 $126.74   10.581%   $140.1504   $140.15   $140.15   $140.20  
  Purchase #3 $191.79   13.450%   $217.5858   $217.59   $217.60   $217.60  
  Purchase #4 $95.35   13.367%   $108.0954   $108.10   $108.10   $108.10  
  Purchase #5 $26.92   12.555%   $30.2998   $30.30   $30.30   $30.30  
  Purchase #6 $229.27   4.735%   $240.1259   $240.13   $240.15   $240.10  
  Purchase #7 $75.50   13.757%   $85.8865   $85.89   $85.90   $85.90  
  7 purchase totals = $796.10       $877.5464   $877.56   $877.60   $877.60