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 $231.39   6.669%   $246.8214   $246.82   $246.80   $246.80  
  Purchase #2 $94.99   3.581%   $98.3916   $98.39   $98.40   $98.40  
  Purchase #3 $15.57   13.644%   $17.6944   $17.69   $17.70   $17.70  
  Purchase #4 $91.79   6.828%   $98.0574   $98.06   $98.05   $98.10  
  Purchase #5 $160.63   11.389%   $178.9242   $178.92   $178.90   $178.90  
  Purchase #6 $239.45   4.675%   $250.6443   $250.64   $250.65   $250.60  
  Purchase #7 $112.66   3.869%   $117.0188   $117.02   $117.00   $117.00  
  7 purchase totals = $946.48       $1,007.5521   $1,007.54   $1,007.50   $1,007.50