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 $230.27   11.407%   $256.5369   $256.54   $256.55   $256.50  
  Purchase #2 $127.66   13.615%   $145.0409   $145.04   $145.05   $145.00  
  Purchase #3 $32.72   6.653%   $34.8969   $34.90   $34.90   $34.90  
  Purchase #4 $240.32   9.321%   $262.7202   $262.72   $262.70   $262.70  
  Purchase #5 $70.59   8.712%   $76.7398   $76.74   $76.75   $76.70  
  Purchase #6 $5.52   8.848%   $6.0084   $6.01   $6.00   $6.00  
  Purchase #7 $140.52   8.760%   $152.8296   $152.83   $152.85   $152.80  
  7 purchase totals = $847.60       $934.7727   $934.78   $934.80   $934.60