I have a document with 8.5x11 facing pages that will be printed on 11x17 paper. Can I span a headline across the facing pages without running into a problem with text being cut off? Or would it be better to create the document as 11x17 instead?
It will not matter if you span your text over facing pages, or if you are using a single page. What matters is how you output, or finish the product.
If you are making a folder with more than 4 pages, you should start thinking about the risk of text being cut of. Generally there is a good rule to not place any letters in the fold. You might also want to increase the kerning slightly between the two letters at each side of the fold, to give some extra space. Also keep low width letters (i, I, l, j) away from the fold, as there is a risk of misalignment resulting in a small width letter disappearing completely.
As a rule I will recommend that you always use facing pages rather than single sheets for folders. This might save you from extra work later in the project.
Any text that falls outside of the printer's printable area (typically .25-.375) will get cut off.
There are a couple of ways you can resolve your issue.
Probably the easiest way around you issues is to check the "Spreads" button in the "General" tab of the print dialog box. This will print the facing pages together on your 11x17 page.
Alternately, you can change your page dimension to 17" x 11" and double your L/R margin in the center of the page to account for the gutter. Then print normally on your 11x17 paper.