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 $121.36   10.604%   $134.229   $134.23   $134.25   $134.20  
  Purchase #2 $38.48   8.189%   $41.6311   $41.63   $41.65   $41.60  
  Purchase #3 $45.73   9.224%   $49.9481   $49.95   $49.95   $49.90  
  Purchase #4 $180.94   12.591%   $203.7222   $203.72   $203.70   $203.70  
  Purchase #5 $66.96   13.735%   $76.157   $76.16   $76.15   $76.20  
  Purchase #6 $85.27   11.779%   $95.314   $95.31   $95.30   $95.30  
  Purchase #7 $141.54   12.268%   $158.9041   $158.90   $158.90   $158.90  
  7 purchase totals = $680.28       $759.9055   $759.90   $759.90   $759.80