Active or inactive, as long as posts deserve to be merited, it is advisable to send your smerits to those posts. You know, merits are for showing appreciation to the content if you find the post meaningful, helpful, and educational to other forum members, and you too can merit them. We don’t have to wait till the OP of such content is an active member since he provides something tangible to the community. You can see that sending some smerits to the post is a good thing to do. However, some users inactiveness may just be for a while before he comes back and sees those merits sent to one of his posts, which may even encourage him to produce more quality contents for the community.
You don’t have to wait to see who the user of a content is before sending merits to them, from newbies to legendarys. Regardless of anyone's rank, if their post is good to be merited, it's advisable to send them merits because if we should say we should consider who the user is, it will make no sense to me, so anybody that is doing a good job for the community is worth meriting.