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 $75.99   6.652%   $81.0449   $81.04   $81.05   $81.00  
  Purchase #2 $162.48   13.237%   $183.9875   $183.99   $184.00   $184.00  
  Purchase #3 $238.96   6.796%   $255.1997   $255.20   $255.20   $255.20  
  Purchase #4 $100.90   12.693%   $113.7072   $113.71   $113.70   $113.70  
  Purchase #5 $166.46   8.688%   $180.922   $180.92   $180.90   $180.90  
  Purchase #6 $34.25   13.749%   $38.959   $38.96   $38.95   $39.00  
  Purchase #7 $163.87   3.866%   $170.2052   $170.21   $170.20   $170.20  
  7 purchase totals = $942.91       $1,024.0255   $1,024.03   $1,024.00   $1,024.00