The simple rule to follow is to merit posts which you consider to be quality. It's good to help newbies rank up, but to do so you should find quality discussions that interests you to merit. Going through the user's post history is a good way to check for merit worthy replies or threads.
Ethically, you shouldn't merit your alts or sell them, the forum has no rules against these, but you can get negative trust if its proven.
I think newbie members generally need a certain amount of time to learn about bitcoin or the forum and make their posts better and more constructive.
That's whlyy I don't think it's necessary to look at their post history so much, but to evaluate each of their posts individually.
if post is good and constructive, why not give it a merit?