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 $123.50   12.783%   $139.287   $139.29   $139.30   $139.30  
  Purchase #2 $174.51   11.143%   $193.9556   $193.96   $193.95   $194.00  
  Purchase #3 $30.37   7.423%   $32.6244   $32.62   $32.60   $32.60  
  Purchase #4 $89.61   5.738%   $94.7518   $94.75   $94.75   $94.80  
  Purchase #5 $173.81   11.462%   $193.7321   $193.73   $193.75   $193.70  
  Purchase #6 $178.98   4.475%   $186.9894   $186.99   $187.00   $187.00  
  Purchase #7 $77.35   8.735%   $84.1065   $84.11   $84.10   $84.10  
  7 purchase totals = $848.13       $925.4468   $925.45   $925.45   $925.50