Some formatting will carry over, but not all. BandHelper's built-in text fields have less formatting capabilities than a full word processor. The font in your files doesn't really matter because the lyrics will display in BandHelper according to the Settings > Appearance > Lyrics Font setting. Also, the margins don't matter because BandHelper doesn't use margins, although you could create the illusion of margins by resizing the document viewer in your layout if you wanted to.
In general I would say that if your plan is to copy content from existing files into BandHelper, don't bother doing any additional formatting in the files, just copy it and go from there. There could be exceptions to that rule if you are encountering some kind of problem when pasting.