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 $249.80   8.275%   $270.471   $270.47   $270.45   $270.50  
  Purchase #2 $104.39   13.812%   $118.8083   $118.81   $118.80   $118.80  
  Purchase #3 $29.51   12.368%   $33.1598   $33.16   $33.15   $33.20  
  Purchase #4 $236.76   9.279%   $258.729   $258.73   $258.75   $258.70  
  Purchase #5 $24.33   12.754%   $27.433   $27.43   $27.45   $27.40  
  Purchase #6 $38.80   12.342%   $43.5887   $43.59   $43.60   $43.60  
  Purchase #7 $249.71   7.754%   $269.0725   $269.07   $269.05   $269.10  
  7 purchase totals = $933.30       $1,021.2623   $1,021.26   $1,021.25   $1,021.30