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 $21.46   5.639%   $22.6701   $22.67   $22.65   $22.70  
  Purchase #2 $11.74   4.570%   $12.2765   $12.28   $12.30   $12.30  
  Purchase #3 $40.79   9.734%   $44.7605   $44.76   $44.75   $44.80  
  Purchase #4 $164.64   4.129%   $171.438   $171.44   $171.45   $171.40  
  Purchase #5 $242.75   5.604%   $256.3537   $256.35   $256.35   $256.40  
  Purchase #6 $7.71   4.718%   $8.0738   $8.07   $8.05   $8.10  
  Purchase #7 $20.90   6.371%   $22.2315   $22.23   $22.25   $22.20  
  7 purchase totals = $509.99       $537.8041   $537.80   $537.80   $537.90