What do you want out of them?
If they are just pets, they need just good forage and/or hay. They will grow slower than grain fed, but being pet wethers, there is no need for fast growth. So long as they are a good body condition, there is nothing 'wrong' with slow growth. As long as they do not stunt from something like coccidia, worms, or illness, they'll just keep growing longer. A good diet of QUALITY forage shouldn't slow them down that much if it is truly a good quality.
If they are slaughter bound and you want them a growthy size by spring to butcher and you want an enhanced fat cover, grain can help get them there, and sooner. If you have them as show animals, it's the same mentality - growthier is better and a better 'cover' on the animal is known as good condition - though they shouldn't be obese (over conditioned) either.
Obviously with wethers the main thing to consider is the calcium: phosphorous ratio to reduce incidence of UC. Some also supplement wethers with ammonium chloride, which is simply a salt that helps acidify urine (and reduce incidence of urinary stones). Personally, I find the by far easiest way to do this is to use a lamb grower feed which comes balanced 2:1 and with an adequate level of AC added in. And its pelleted so they can't be choosy. A specific 'goat feed' that is balanced 2:1 and has AC would also be good, but those were over a dollar more per bag so I went with the lamb grower.
Free choice minerals, baking soda, and water should always be part of a diet. Always feed to body condition - if they seem a little thin (less muscling/fat cover than ideal) then perhaps a couple handfuls of grain as a treat will help them gain a bit. Sometimes you can also increase their hay/forage quality and effect the change you want. You do not want or need 'fat' animals if they are pets... for long life, too much fat greatly effects their long term health.