Bleeds are necessary once you have any sort of colour that is placed right at the edge of the paper. Industry standard of bleeds are 1/8", so make sure any images/background boxes are constructed 1/8" outside of your live area. One thing about magazines/books that you need to look for is whether this is saddle-stitched (2 staples on the spine) or perfect bound (uses adhesive/glue). If it is saddle-stitched, the cover should be fine just as it is, however, if it is perfect bound, you will need to create a gutter/spine area in your cover artwork, which means that when you are constructing the cover, it should be slightly larger than the inside content pages. This depends on the thickness of the magazine (how many content pages there will be), if its not too thick then I think saddle-stitched will be suffice. I've made a habit of packaging the entire InDesign file, but also making sure I export a high-resolution PDF file (with bleeds) and copy it into the packaged folder, zip the folder, then send the entire zipped file to the printer. I would think most printers would use the PDF file, but if they were to choose the InDesign file, they would still have a PDF to refer to. This is how I was taught in school, and this is what my workplace asks of clients (though we only get what we need like... 1 out of 1000 files). But I do echo what oddball said, there are printers that like to do things their own way. Contacting them would be a good start. Also, ask for a printed proof! There may be more details, but can't think of them as of now.