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 $240.32   12.256%   $269.7736   $269.77   $269.75   $269.80  
  Purchase #2 $85.67   11.652%   $95.6523   $95.65   $95.65   $95.70  
  Purchase #3 $245.59   9.495%   $268.9088   $268.91   $268.90   $268.90  
  Purchase #4 $145.89   12.167%   $163.6404   $163.64   $163.65   $163.60  
  Purchase #5 $7.64   11.123%   $8.4898   $8.49   $8.50   $8.50  
  Purchase #6 $68.89   10.599%   $76.1917   $76.19   $76.20   $76.20  
  Purchase #7 $140.56   4.319%   $146.6308   $146.63   $146.65   $146.60  
  7 purchase totals = $934.56       $1,029.2874   $1,029.28   $1,029.30   $1,029.30