It's true that a more oblique or indirect expression can sound more polite. In approximate order of increasing politeness/deference, we might list
Answer me this:
I would ask you this:
I have a question:
I had a question:
I might have a question:
Would you be willing to answer a question?
I wonder if you might possibly be willing to consider a question?
And so on. Some might consider "I had a question:" a gentler and more polite expression than "I have a question:" because it implies that the question hasn't been constantly on your mind; it arose once and is incidental.
When communicating in 2018, however, "I have a question:" is almost always going to be appropriate in business and personal communication and is generally the simplest way to proceed. It's suitably deferential and polite because you're submitting a query to the other person and yielding the floor for them to answer (i.e., you're giving them the opportunity to speak). It can sound obsequious or overly deferential or wordily self-indulgent to use a longer and more indirect expression such as the last few in the list above.