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 $248.92   8.581%   $270.2798   $270.28   $270.30   $270.30  
  Purchase #2 $191.76   9.475%   $209.9293   $209.93   $209.95   $209.90  
  Purchase #3 $71.39   7.294%   $76.5972   $76.60   $76.60   $76.60  
  Purchase #4 $236.49   12.198%   $265.3371   $265.34   $265.35   $265.30  
  Purchase #5 $206.33   8.736%   $224.355   $224.36   $224.35   $224.40  
  Purchase #6 $54.99   13.848%   $62.605   $62.61   $62.60   $62.60  
  Purchase #7 $218.78   13.239%   $247.7443   $247.74   $247.75   $247.70  
  7 purchase totals = $1,228.66       $1,356.8477   $1,356.86   $1,356.90   $1,356.80