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 $100.51   7.857%   $108.4071   $108.41   $108.40   $108.40  
  Purchase #2 $31.78   10.459%   $35.1039   $35.10   $35.10   $35.10  
  Purchase #3 $154.75   11.288%   $172.2182   $172.22   $172.20   $172.20  
  Purchase #4 $158.38   8.122%   $171.2436   $171.24   $171.25   $171.20  
  Purchase #5 $105.39   8.711%   $114.5705   $114.57   $114.55   $114.60  
  Purchase #6 $14.99   9.612%   $16.4308   $16.43   $16.45   $16.40  
  Purchase #7 $24.36   8.494%   $26.4291   $26.43   $26.45   $26.40  
  7 purchase totals = $590.16       $644.4032   $644.40   $644.40   $644.30