Should you always get a tax identification number for a retirement plan?

My question is should you always get a tax identification number when you are setting up a new retirement plan? I setup a lot of new plans every year and a lot of them are with recordkeepers and my understanding is if the TIN isn't used the IRS will shut it down so a lot of times I haven't been getting a TIN and have just been using the EIN of the business. I know this is technically not accurate since the assets are not the employer's assets but from what I have seen I think this is quite common. Most recordkeepers I work with have never asked me for a Plan TIN for a plan and have just used the EIN of the business. 

I also setup a lot of solo 401(k)'s and have been doing the mega backdoor internal Roth conversion of after-tax employee money and these types of plans typically in my experience are setup in self-directed brokerage accounts so when I do the 1099 for the conversion I will have to have a TIN for the plan so these plans I have usually been applying for a plan TIN. 

Is it recommended to always get a plan TIN no matter what? Or from a practical standpoint can you just get one when you know you will use it on 1099's, etc. 

I agree with your take - don't need it if a plan is with a recordkeeper and they are doing tax reporting (1099-Rs,945s). Do need it otherwise.

It has been a while since I thought about this.  My recollection from the early days of ERISA and how one should report income centered on avoiding confusion at tax time. 

I counseled clients starting new plans to get a separate trust identification number for new trusts.  I have always told the new plan sponsor that this approach will ensure the trust's income does not get reported as income to the plan sponsor which could happen if the plan sponsor's EIN is used.  I cannot think of a plan sponsor client that has experienced this kind of confusion, but that was the reason.  Perhaps my clients didn't have this problem because they used a separate TIN.

The exception to the separate TIN rule applied in cases where the trustee was a corporate trustee that used its trust identification number to hold and report earnings. 

More modernly, this issue hasn't come up for me.  I have no single employee 401(k) plans

The Employer Identification Number of the plan sponsor plus the three digit plan number were used for reporting in other than the investment matters.

In my TPA days I often saw trust income reported as being taxable income to the plan sponsor and eliciting an IRS response, so I think this is a real concern.  My client base was physicians and small entities so I imagine they were likely to be audit targets!

Is there any downside to getting a TIN?

If no, then why not get one always?

We used to get them for our non-product plans, but very often many years would go by without a distribution, and when one would finally happen, the TIN would be 'stale' and we'd get a notice that said TIN was not associated with the trust any longer - the IRS would assume it was no longer in use and re-assign it.  Also, and I don't remember the details on this, but since withholding rules have changed, it is not as likely that there would be any crossover confusion between the plan and the employer (I really don't remember exactly what that was all about - I remember a CPA told me), so there's another reason not to get it.  And we make any cash distributions get paid out through a distribution service (Penchecks) so they do withholding... between all those, I don't see the point any more.