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 $231.73   11.227%   $257.7463   $257.75   $257.75   $257.70  
  Purchase #2 $183.74   11.583%   $205.0226   $205.02   $205.00   $205.00  
  Purchase #3 $242.99   5.619%   $256.6436   $256.64   $256.65   $256.60  
  Purchase #4 $50.45   13.526%   $57.2739   $57.27   $57.25   $57.30  
  Purchase #5 $110.67   8.358%   $119.9198   $119.92   $119.90   $119.90  
  Purchase #6 $67.92   8.214%   $73.4989   $73.50   $73.50   $73.50  
  Purchase #7 $22.35   8.347%   $24.2156   $24.22   $24.20   $24.20  
  7 purchase totals = $909.85       $994.3207   $994.32   $994.25   $994.20