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 $21.84   13.273%   $24.7388   $24.74   $24.75   $24.70  
  Purchase #2 $184.39   8.314%   $199.7202   $199.72   $199.70   $199.70  
  Purchase #3 $232.42   4.427%   $242.7092   $242.71   $242.70   $242.70  
  Purchase #4 $150.73   4.518%   $157.54   $157.54   $157.55   $157.50  
  Purchase #5 $236.38   13.642%   $268.627   $268.63   $268.65   $268.60  
  Purchase #6 $32.68   12.150%   $36.6506   $36.65   $36.65   $36.70  
  Purchase #7 $131.75   12.866%   $148.701   $148.70   $148.70   $148.70  
  7 purchase totals = $990.19       $1,078.6868   $1,078.69   $1,078.70   $1,078.60