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 $27.31   13.418%   $30.9745   $30.97   $30.95   $31.00  
  Purchase #2 $142.53   5.416%   $150.2494   $150.25   $150.25   $150.20  
  Purchase #3 $28.31   7.510%   $30.4361   $30.44   $30.45   $30.40  
  Purchase #4 $122.34   12.615%   $137.7732   $137.77   $137.75   $137.80  
  Purchase #5 $106.69   7.466%   $114.6555   $114.66   $114.65   $114.70  
  Purchase #6 $14.29   7.854%   $15.4123   $15.41   $15.40   $15.40  
  Purchase #7 $64.90   11.792%   $72.553   $72.55   $72.55   $72.60  
  7 purchase totals = $506.37       $552.0540   $552.05   $552.00   $552.10