Guys like Hiura, who get drafted in the first round, are locks to sign out of the draft. It is rare to waste a high pick on a questionable sign. The later rounds, though bring more questions of who will sign. That sign-ability is a huge part of how draft prospects are assessed. If the sign-ability is low, so is the draft stock.
If you have ever watched day two of the MLB Draft, you will hear the draft experts talk about the sign-ability aspect. Generally, the top ten rounds are all players who have a high likelihood to ink with the team.