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 $51.97   11.671%   $58.0354   $58.04   $58.05   $58.00  
  Purchase #2 $15.59   5.675%   $16.4747   $16.47   $16.45   $16.50  
  Purchase #3 $98.64   4.802%   $103.3767   $103.38   $103.40   $103.40  
  Purchase #4 $35.36   13.263%   $40.0498   $40.05   $40.05   $40.00  
  Purchase #5 $231.50   5.789%   $244.9015   $244.90   $244.90   $244.90  
  Purchase #6 $241.39   4.666%   $252.6533   $252.65   $252.65   $252.70  
  Purchase #7 $46.83   7.193%   $50.1985   $50.20   $50.20   $50.20  
  7 purchase totals = $721.28       $765.6899   $765.69   $765.70   $765.70