This has been briefly discussed in chat here and there over the months, and it currently isn't on-topic either way.
Asking for a build from scratch
This is definitely the more off-topic question of the two you suggest. Although it has good intentions, this site only accepts questions asking for a single, specific piece of hardware. This means that asking for recommendations for entire computer builds (i.e., many different types of hardware) is easily off-topic.
Only a single type of hardware can be asked about in a question — that is the basis of the scope of this site.
Asking for a review of your build
This seems like it could work, and that's exactly why it's been discussed in the past, but alas it doesn't (at least not yet). It isn't really asking for a single product recommendation, but it also isn't clearly off-topic by definition — asking about a current build could possibly be read as asking for a recommendation. It's a gray area we currently have, and it's best to avoid these areas.
Additionally, as a few people have pointed out recently (example), a build review/the system in question could be seen as one piece of hardware. Going about it this way has its pros and cons though. Would they elicit too many broad answers? How would the answers be judged for quality? There are several downfalls to this approach, yet there is a chance it could work nonetheless.
If you really want to ask for or about a build, for now the best way is to ask one question for each component. This way each question is about a specific piece of hardware.