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 $17.48   13.681%   $19.8714   $19.87   $19.85   $19.90  
  Purchase #2 $30.37   13.394%   $34.4378   $34.44   $34.45   $34.40  
  Purchase #3 $133.35   8.752%   $145.0208   $145.02   $145.00   $145.00  
  Purchase #4 $42.54   8.699%   $46.2406   $46.24   $46.25   $46.20  
  Purchase #5 $111.30   5.441%   $117.3558   $117.36   $117.35   $117.40  
  Purchase #6 $38.80   7.309%   $41.6359   $41.64   $41.65   $41.60  
  Purchase #7 $78.39   7.593%   $84.3422   $84.34   $84.35   $84.30  
  7 purchase totals = $452.23       $488.9045   $488.91   $488.90   $488.80