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 $241.25   3.660%   $250.0798   $250.08   $250.10   $250.10  
  Purchase #2 $125.93   7.355%   $135.1922   $135.19   $135.20   $135.20  
  Purchase #3 $68.70   11.776%   $76.7901   $76.79   $76.80   $76.80  
  Purchase #4 $229.25   7.289%   $245.96   $245.96   $245.95   $246.00  
  Purchase #5 $14.38   4.484%   $15.0248   $15.02   $15.00   $15.00  
  Purchase #6 $67.37   9.180%   $73.5546   $73.55   $73.55   $73.60  
  Purchase #7 $237.42   9.148%   $259.1392   $259.14   $259.15   $259.10  
  7 purchase totals = $984.30       $1,055.7407   $1,055.73   $1,055.75   $1,055.80