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 $61.44   6.478%   $65.4201   $65.42   $65.40   $65.40  
  Purchase #2 $32.84   11.487%   $36.6123   $36.61   $36.60   $36.60  
  Purchase #3 $158.63   5.721%   $167.7052   $167.71   $167.70   $167.70  
  Purchase #4 $114.72   7.401%   $123.2104   $123.21   $123.20   $123.20  
  Purchase #5 $10.70   8.118%   $11.5686   $11.57   $11.55   $11.60  
  Purchase #6 $21.40   10.399%   $23.6254   $23.63   $23.65   $23.60  
  Purchase #7 $77.56   7.647%   $83.491   $83.49   $83.50   $83.50  
  7 purchase totals = $477.29       $511.6330   $511.64   $511.60   $511.60