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 $63.93   3.163%   $65.9521   $65.95   $65.95   $66.00  
  Purchase #2 $7.29   12.613%   $8.2095   $8.21   $8.20   $8.20  
  Purchase #3 $68.44   8.213%   $74.061   $74.06   $74.05   $74.10  
  Purchase #4 $114.83   12.574%   $129.2687   $129.27   $129.25   $129.30  
  Purchase #5 $66.38   7.357%   $71.2636   $71.26   $71.25   $71.30  
  Purchase #6 $210.47   9.610%   $230.6962   $230.70   $230.70   $230.70  
  Purchase #7 $12.69   13.641%   $14.421   $14.42   $14.40   $14.40  
  7 purchase totals = $544.03       $593.8721   $593.87   $593.80   $594.00