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 $249.90   10.472%   $276.0695   $276.07   $276.05   $276.10  
  Purchase #2 $53.48   5.227%   $56.2754   $56.28   $56.30   $56.30  
  Purchase #3 $238.45   8.169%   $257.929   $257.93   $257.95   $257.90  
  Purchase #4 $14.77   13.215%   $16.7219   $16.72   $16.70   $16.70  
  Purchase #5 $209.32   9.227%   $228.634   $228.63   $228.65   $228.60  
  Purchase #6 $242.74   6.432%   $258.353   $258.35   $258.35   $258.40  
  Purchase #7 $247.36   11.532%   $275.8856   $275.89   $275.90   $275.90  
  7 purchase totals = $1,256.02       $1,369.8684   $1,369.87   $1,369.90   $1,369.90