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 $6.44   5.491%   $6.7936   $6.79   $6.80   $6.80  
  Purchase #2 $139.88   12.624%   $157.5385   $157.54   $157.55   $157.50  
  Purchase #3 $20.82   7.394%   $22.3594   $22.36   $22.35   $22.40  
  Purchase #4 $89.56   6.330%   $95.2291   $95.23   $95.25   $95.20  
  Purchase #5 $30.95   7.598%   $33.3016   $33.30   $33.30   $33.30  
  Purchase #6 $117.45   5.500%   $123.9098   $123.91   $123.90   $123.90  
  Purchase #7 $94.64   6.659%   $100.9421   $100.94   $100.95   $100.90  
  7 purchase totals = $499.74       $540.0741   $540.07   $540.10   $540.00