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 $17.75   3.604%   $18.3897   $18.39   $18.40   $18.40  
  Purchase #2 $146.37   12.251%   $164.3018   $164.30   $164.30   $164.30  
  Purchase #3 $133.71   13.196%   $151.3544   $151.35   $151.35   $151.40  
  Purchase #4 $84.60   7.819%   $91.2149   $91.21   $91.20   $91.20  
  Purchase #5 $242.93   7.589%   $261.366   $261.37   $261.35   $261.40  
  Purchase #6 $28.80   11.754%   $32.1852   $32.19   $32.20   $32.20  
  Purchase #7 $140.48   11.243%   $156.2742   $156.27   $156.25   $156.30  
  7 purchase totals = $794.64       $875.0862   $875.08   $875.05   $875.20