Mike, you are correct in that the issue key to the question is whether the "relation-back" doctrine applies to non-charitable gifts. Bacchus, Joan W. vs US, 57 AFTR 2d 86-1519 applied the doctrine to exclude the checks cashed after death. The Seveth Circuit, Daniel F. McCarthy vs. US 59 AFTR 2d 87-1193 and the Tax Court in Gagliardi, Joseph vs Commissioner, 89 TC 1207 said it didn't apply to non charitable gifts. In the later cases they distinguish between the application of the "relation-back" doctrine for checks in discharge of a bon afide legal obligation or charitable gift verses a non charitable gift.

The IRS also takes the position that the date cashed is controlling.

It could be a point of litigation with IRS so look at your jurisdiction and discuss the position with you client.