There's another current discussion of using "icons" (on Ancestry this requires a photograph/picture that serves as an icon) to designate direct lines, immigrants, any and everything that someone wants to highlight. The use of these "icons" generates hints to other trees (if your tree is public), some are of the opinion that it's a lot of clutter.
There is also the question of how these "icons" will transfer to a desk-top genealogy program, ie, downloading and creating a GED from your Ancestry.com tree. Unless you're using FTM, those pics/icons won't transfer.
The old and tried and true method to designate direct lines, is to CAP the surname of the direct ancestor. This translates well to all other programs, and most recognize the CAPPED surnames are the direct lines, without additional explanation. Keep it lean and mean.