I've had my BPs for 2 years and always gave them tap water; never had any issues. I did recently start giving them distilled water within the last 3 months, for no particular reason other than we had it on hand. I haven't noticed any differences in their behavior or overall health since the change. I may keep with the distilled water since it takes so long for them to go through a couple gallons anyway it's not really an inconvenience, but there has been nothing that has made me feel like tap water was not safe to use. However, that's only with 2 years of use so who knows what problems it might cause down the line? Maybe another more seasoned owner will be able to give some more insight.
I will note that not all tap water is the same, some areas may need more treatment than other and in turn have more chemicals. If you want to be safe, I'd suggest using distilled water anyway. I get mine from Walmart at $1.15 a gal and a gallon lasts about a month between 10 snakes and 2 geckos, so it should last you quite some time.
Congrats on the new (or soon to come?) purchase! Ball Pythons are awesome and terribly addictingSo make sure you share some pics!