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 $245.33   13.789%   $279.1586   $279.16   $279.15   $279.20  
  Purchase #2 $112.92   8.775%   $122.8287   $122.83   $122.85   $122.80  
  Purchase #3 $158.46   10.181%   $174.5928   $174.59   $174.60   $174.60  
  Purchase #4 $108.87   9.682%   $119.4108   $119.41   $119.40   $119.40  
  Purchase #5 $118.56   9.144%   $129.4011   $129.40   $129.40   $129.40  
  Purchase #6 $50.50   8.189%   $54.6354   $54.64   $54.65   $54.60  
  Purchase #7 $31.71   9.355%   $34.6765   $34.68   $34.70   $34.70  
  7 purchase totals = $826.35       $914.7039   $914.71   $914.75   $914.70