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 $242.98   11.752%   $271.535   $271.54   $271.55   $271.50  
  Purchase #2 $61.31   13.380%   $69.5133   $69.51   $69.50   $69.50  
  Purchase #3 $167.29   5.523%   $176.5294   $176.53   $176.55   $176.50  
  Purchase #4 $249.46   8.839%   $271.5098   $271.51   $271.50   $271.50  
  Purchase #5 $98.76   7.507%   $106.1739   $106.17   $106.15   $106.20  
  Purchase #6 $132.63   12.368%   $149.0337   $149.03   $149.05   $149.00  
  Purchase #7 $196.67   13.843%   $223.895   $223.90   $223.90   $223.90  
  7 purchase totals = $1,149.10       $1,268.1901   $1,268.19   $1,268.20   $1,268.10