Generally speaking a deceased person's unpaid expenses, including income taxes, are to be paid out of his or her probate estate. However, estate, gift, and generation skipping transfer taxes are doled out pro rata among the beneficiaries / heirs of the decedent, e.g., beneficiary of life insurance policies, IRAs, or other retirement accounts, and the surviving joint tenants of joint tenancy assets. That being said, it is fairly common for those with Wills and Revocable Living Trusts to specify that the expenses and taxes are to be paid out of the residue of the estate or trust estate.
Although many trusts specify that expenses and taxes are to be paid out of the residue of the trust estate, one should consider whether that is a fair burden on the residuary beneficiary(ies) of the trust. If a large portion of your assets passes outside of your trust, for example, by way of a large retirement account, it may not be fair for the residuary beneficiary(ies) of your trust to be responsible for the taxes related to that retirement account. On the other hand, if the beneficiary of the retirement account and the residuary beneficiary(ies) of your trust are the same people, it would not matter since the same people are ultimately responsible for the taxes.