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 $35.47   8.223%   $38.3867   $38.39   $38.40   $38.40  
  Purchase #2 $164.26   13.171%   $185.8947   $185.89   $185.90   $185.90  
  Purchase #3 $229.63   4.299%   $239.5018   $239.50   $239.50   $239.50  
  Purchase #4 $12.46   4.628%   $13.0366   $13.04   $13.05   $13.00  
  Purchase #5 $74.57   7.529%   $80.1844   $80.18   $80.20   $80.20  
  Purchase #6 $241.36   13.324%   $273.5188   $273.52   $273.50   $273.50  
  Purchase #7 $166.97   9.442%   $182.7353   $182.74   $182.75   $182.70  
  7 purchase totals = $924.72       $1,013.2583   $1,013.26   $1,013.30   $1,013.20