John, there is nothing standard for validating dimensions during data entry. It becomes a matter of 'when and what to validate'.
The best we have is the posting preview.
If you think about the validation, first you must find every general ledger account that will be affected during the posting and then review their dimension requirements to determine what validation is required. This is why the validation is done during posting preview.
I believe a customization could be built that you could execute before releasing your purchase order, maybe even before, sending for approval. The customization would basically replicate the preview posting but only give errors related to dimension issues.
