It depends on your actual stuff. A lot of bikes have double eyelets on the rear, and they are useful for attaching your rack forward, and the rear fender behind the reasons for that are in descending order:
- If the fender was attached with the rack, it would block access to the place you put your pannier tie, depending on position and number of ties;
- Any time you attach the rack eyelets and the fender eyelets to the same bolt, there is a chance of everything coming loose if your fender eyelet is the usual bent piece of wire. it can simply spread apart. There are options to separate placement like washers, or my favorite, welding the wire to a solid piece of stainless so that it can be bolted on as hard as you want. Also, some fenders can be attached to eyelets on the rear of the rack.
- the dual eyelet position allows some flexibility for positioning the rack weight over the axle.
On the front, most racks need to be attached to the same lower single eyelet that comes on forks with only one eyelet, and you have to attach the fender there also, or to the rack if that will work. While the upper braze-on is not required for most racks or fenders, a braze-on higher on the fork is very useful, and it isn't likely to be provided on non-touring equipped bikes which would probably all have the second braze-on anyway.