I can’t think of any really important reason why you couldn’t use Xcode 10.1 for new releases. If that is the version you have done a lot of development and testing on, then you might be more comfortable from a project management perspective to not change just before a release. But I don’t think that’s a very strong argument to avoid Xcode 10.3 for any significant length of time.

I don’t think much changed between iOS 12.1 and 12.4 other than bug fixes. But your users who are on 12.x are probably on 12.4. So if you’re testing using Xcode 10.1 simulator (as opposed to a real device), then you’re not testing on the version your customers are using. That’s a small risk.

Another good reason to use Xcode 10.3 instead of 10.1 is that Xcode itself presumably has some bug fixes.

When Apple started requiring 10.1 I suspect a lot of people with old apps will have had to do a lot of work to move from Xcode 9 (or 8!) and also handle all the new devices and screen sizes. That would be a lot of work to modernise an old app. Moving from Xcode 10.1 to 10.3 is a much smaller change and much less risky than moving between major versions.