There is a difference in meaning between the two words:
In your particular sentences this difference in meaning has essentially the same result: the reason why you are asking the question is most likely a logical consequence of the prior statement:
Mia demando estas do ĉu ĝi validas aŭ ne
Therefore my question is…
Mia demando estas tial ĉu ĝi validas aŭ ne
That's why my question is…
This boils down to the same thing. In a lot of cases do and tial can be used more or less interchangeably, because if there is a reason that something happens, it is often a logical consequence as well; conversely, if something is a logical consequence, then that fact is a good reason for why it happens.
At the moment I cannot think of a good example to contrast the two meanings better, or where using one word would really give an essentially different sentence than when you would use the other. I'll update my answer if I think of anything.