It is best to place the lettering in the lower third of the card. This allows the main motif to find space above the text stamp.
Make sure to stamp the lettering as straight as possible and to keep the same distance to the left and right edges of the card. It becomes even more symmetrical if the distances between the main motif, text stamp and lower edge of the card are identical.
This sounds harder than it is - trust your eye and with a little practice you'll get the hang of it in no time.