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 $12.27   5.745%   $12.9749   $12.97   $12.95   $13.00  
  Purchase #2 $5.82   3.815%   $6.042   $6.04   $6.05   $6.00  
  Purchase #3 $32.94   11.383%   $36.6896   $36.69   $36.70   $36.70  
  Purchase #4 $155.71   11.296%   $173.299   $173.30   $173.30   $173.30  
  Purchase #5 $38.39   11.483%   $42.7983   $42.80   $42.80   $42.80  
  Purchase #6 $204.46   13.427%   $231.9128   $231.91   $231.90   $231.90  
  Purchase #7 $8.47   5.392%   $8.9267   $8.93   $8.95   $8.90  
  7 purchase totals = $458.06       $512.6433   $512.64   $512.65   $512.60