It’s ok to choose the most suitable definition of a credible region for your problem. The Wikipedia entry explains it well. Your latter suggestion sounds preferable for your case, though it would be a 95% interval.
An obvious situation where your choice matters is when your posterior distribution is multimodel.
(picture nabbed from here)
While both intervals contain 90% of the posterior density, forming two intervals is usually preferable, since
choosing the single interval in the picture covers a region of negligible posterior density.