Properly assembled, it is not something to worry about. There are literally tens of millions of two piece toilets installed around the country. Most will never experience a leak in their normal lifetime.
Now, is the tank to bowl junction an area that can leak, well of course, but the occurrences are so infrequent as to not worry about. Many other important considerations such as price, style, availability of repair parts, and flush performance give you enough to think about!