Will it ruin it?

Oh, no...it wont ruin it at all. Its like finding out more stories about a dear old friend.

No. When I previewed Saving Mr. Banks (via a D23 invitation), the theater showed Mary Poppins directly after it.

I think Saving Mr. Banks may have enhanced the viewing of Mary Poppins, since you got to start picking out the small details between both films.

No. Much of the film is fictionalized and characters are made better/worse than they were in real life. This is nothing more than a story to be taken with a grain of salt. Most of the bad stuff comes from the butthurt mind of Richard Sherman who painted Travers as a monster, even though almost everyone else's accounts of her and actual audio of their sessions shows that she really was not as bad as he wants people to believe.

I have not seen Mary Poppins (1964) and, frankly, Saving Mr Banks did not create any desire in me to watch Mary Poppins. Seems like one of those cheesy 50s-60s musicals.

If anything it'll galvanize your opinion of how Disney turned a halfway forgettable book by a despicable, hateful woman into a timeless thing of joy.

If anything I think that the popularity of 'Mary Poppins has increased since this was made. I always like to see classic songs being shown at their composing stage in movies like this.