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 $240.96   11.844%   $269.4993   $269.50   $269.50   $269.50  
  Purchase #2 $188.27   13.594%   $213.8634   $213.86   $213.85   $213.90  
  Purchase #3 $227.63   13.585%   $258.5535   $258.55   $258.55   $258.60  
  Purchase #4 $117.63   12.601%   $132.4526   $132.45   $132.45   $132.50  
  Purchase #5 $121.78   5.702%   $128.7239   $128.72   $128.70   $128.70  
  Purchase #6 $132.29   8.213%   $143.155   $143.16   $143.15   $143.20  
  Purchase #7 $56.90   6.833%   $60.788   $60.79   $60.80   $60.80  
  7 purchase totals = $1,085.46       $1,207.0357   $1,207.03   $1,207.00   $1,207.20