When asking a question
Generally, is more effective to ask the question in a way that encourages the type of answer that you are looking for instead of the type of person you want to answer as it distracts from the main point and identity is generally anonymous here anyway.
Example: when asking a question about Korean etiquette it is completely understandable to want answers specifically from people who have lived in Korea and experienced the etiquette first hand. Fairly obviously, asking a general etiquette question, and then specifying that you want only the perspective of a Korean person is not the most effective way to get the answers you are hoping for. Instead simply mentioning that the question is about Korean etiquette and including the Korea tag is far clearer and will have a better chance to result in the type of answer you are looking for.
There are some grey areas, but as a rule of thumb if it is important enough for answers to be from a specific point of view you should be able to ask the question in a way that prompts for the type of answer you want. Not the type of answerer you want.
When commenting on a question
The asker of a question generally has more context and understanding of the scenario they are describing so rather than making a presumptuous answer, asking for OP's point of view is often the most reliable source for this information. Contrary to sphennings' answer I would actually say this helps to stop questions from being left as bad subjective questions. It stops answers from having to guess which stops subjective argumentative discussion from happening. Sure, it would be ideal for all of our situations to be made up of clear objective facts but that is not always the case and when a question does have a subjective component it is best to establish from OP's point of view exactly where everything stands so that answerers can use it as a starting point. They may like to account for the other possibilities, or even give their reasons why they disagree and answer accordingly but the point is that a good answer would give those reasons in the answer rather than just making a series of their own assumptions and arguing about it in chat. So, I would say that asking for OP's opinion on a relevant topic in comments can actually be an important part of encouraging clearer questions and when done right can turn bad subjective questions into good subjective ones.
We encourage answers to be as objective as possible, writing from your own point of view is fine (for example you could reference personal experiences you have had) but injecting your opinion without explanation is discouraged. Backing up your answer is a good way to improve your answers and minimise their subjectivity.