I have been using an FT817ND as an IF rig for the microwave bands on 144 and 432 MHz. I do not have the TCXO installed. I have never had any problem with its stability or accuracy.
I have heard you do need the TCXO for VHF/UHF.? I don't think you do for HF as I am running it even with WSPR it seems to work fine on HF.? However, the Chinese knock offs generate more phase noise.