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 $56.77   12.173%   $63.6806   $63.68   $63.70   $63.70  
  Purchase #2 $145.38   8.867%   $158.2708   $158.27   $158.25   $158.30  
  Purchase #3 $33.50   9.788%   $36.779   $36.78   $36.80   $36.80  
  Purchase #4 $54.25   8.617%   $58.9247   $58.92   $58.90   $58.90  
  Purchase #5 $234.75   10.719%   $259.9129   $259.91   $259.90   $259.90  
  Purchase #6 $92.27   10.771%   $102.2084   $102.21   $102.20   $102.20  
  Purchase #7 $221.92   7.851%   $239.3429   $239.34   $239.35   $239.30  
  7 purchase totals = $838.84       $919.1193   $919.11   $919.10   $919.10