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 $238.63   6.104%   $253.196   $253.20   $253.20   $253.20  
  Purchase #2 $66.28   6.437%   $70.5464   $70.55   $70.55   $70.50  
  Purchase #3 $89.31   5.224%   $93.9756   $93.98   $94.00   $94.00  
  Purchase #4 $211.71   4.783%   $221.8361   $221.84   $221.85   $221.80  
  Purchase #5 $13.77   12.558%   $15.4992   $15.50   $15.50   $15.50  
  Purchase #6 $44.71   9.306%   $48.8707   $48.87   $48.85   $48.90  
  Purchase #7 $127.33   7.483%   $136.8581   $136.86   $136.85   $136.90  
  7 purchase totals = $791.74       $840.7821   $840.80   $840.80   $840.80