Grouping feature checks that the sum of all selected lines == 0, which is often not the case as the values are float. For that, our approach is to round the values with the precision of the accounting currency, since these precisions are usually small (typically 0, 2 or 3), we don't have problems with rounding. Using the section currency is not just a workaround for rounding, it's also correct because we don't consider more precise amounts in accounting transaction lines.
The problem with this approach was for the case where no accounting currency is set on the section organisation, in that case we did not round and this sometimes led to "grouping is impossible" errors that are hard to find for users. At this level it's better to use a default rounding precision that would make it possible to use the grouping feature even when section currency is not set.