Choose declared value by looking at what the card could realistically be worth in the condition outcomes you are submitting for. Do not use only the highest grade, and do not guess low just to force a cheaper service level. Declared value can affect service eligibility, insurance, liability, and upcharges. If you submit through a local grading shop, ask whether you or the shop chooses the value and how the number is documented.
A practical approach is to compare recent sales for the same card raw, in a realistic mid-grade, and in the optimistic grade you hope for. Then choose a value that follows the grader’s rules and protects you if the card is lost or damaged. The declared value guide explains the tradeoffs in more detail. If a shop dismisses declared value as unimportant, slow down and ask for the policy. The number is not a promise that the card will sell for that amount, but it is part of the risk contract around the submission.