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 $9.66   3.434%   $9.9917   $9.99   $10.00   $10.00  
  Purchase #2 $161.26   12.120%   $180.8047   $180.80   $180.80   $180.80  
  Purchase #3 $193.38   11.844%   $216.2839   $216.28   $216.30   $216.30  
  Purchase #4 $37.76   4.567%   $39.4845   $39.48   $39.50   $39.50  
  Purchase #5 $246.25   12.696%   $277.5139   $277.51   $277.50   $277.50  
  Purchase #6 $175.56   10.277%   $193.6023   $193.60   $193.60   $193.60  
  Purchase #7 $196.61   11.741%   $219.694   $219.69   $219.70   $219.70  
  7 purchase totals = $1,020.48       $1,137.3750   $1,137.35   $1,137.40   $1,137.40