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 $248.55   11.728%   $277.6999   $277.70   $277.70   $277.70  
  Purchase #2 $98.59   6.429%   $104.9284   $104.93   $104.95   $104.90  
  Purchase #3 $76.46   12.298%   $85.8631   $85.86   $85.85   $85.90  
  Purchase #4 $46.92   11.395%   $52.2665   $52.27   $52.25   $52.30  
  Purchase #5 $236.58   12.313%   $265.7101   $265.71   $265.70   $265.70  
  Purchase #6 $156.35   11.672%   $174.5992   $174.60   $174.60   $174.60  
  Purchase #7 $25.73   6.811%   $27.4825   $27.48   $27.50   $27.50  
  7 purchase totals = $889.18       $988.5497   $988.55   $988.55   $988.60