Kami,
Shields will not be retained with the default labeling for Feature layers in ArcGIS online. labels are still very basic.
A Hosted Tile layer will support the shields and any scale visibility you set in desktop.
As a rule of thumb when considering Tiles or Features,
- Use Tiles to deliver cartographically rich map.
- Use Features to deliver data and attributes.
If it helps, I can add more technical explanation, that labels and their placement require a good deal of processing power that are suited to desktop environments, In a web browser however the same types of processing would slow down the webpage, thus we see fairly basic labeling of features. Now with Tiles, its essentially a collection of lightweight image files. All the heavy lifting used to create a detailed map is performed ahead of time, and the results saved at tile images. So the browsers jod is much simpler, Just request and display the image tiles for this area.
On a side note: Tile layers may appear to be only Static, but they can be configured to display popups from existing feature layers.
-Adam