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 $209.85   11.527%   $234.0394   $234.04   $234.05   $234.00  
  Purchase #2 $63.74   10.684%   $70.55   $70.55   $70.55   $70.60  
  Purchase #3 $187.83   3.145%   $193.7373   $193.74   $193.75   $193.70  
  Purchase #4 $176.77   11.732%   $197.5087   $197.51   $197.50   $197.50  
  Purchase #5 $97.48   12.104%   $109.279   $109.28   $109.30   $109.30  
  Purchase #6 $129.82   8.528%   $140.891   $140.89   $140.90   $140.90  
  Purchase #7 $30.65   5.242%   $32.2567   $32.26   $32.25   $32.30  
  7 purchase totals = $896.14       $978.2621   $978.27   $978.30   $978.30