Are there any disadvantages in registering for an EIN for a single-member LLC?

There's no real downside, in fact not much of anything happens. But it is a necessary step in creating a business as it's own entity, which anyone operating anything other than a sole-proprietorship will need. No reason not to do it and get it out of the way. You may also need Secretary of State paperwork showing LLC registration (which requires EIN) to open the bank account.

Also, if you intend to build credit as the business, make a few purchases from companies that report transactions to get your company officially "on record" from a credit perspective.

Yes, you should make the account owned by your legal entity, not you as an individual. This helps avoid what's known as "co-mingling of funds", which helps you limit your personal liability if your company were ever sued.

However, in most cases, you DO NOT want to be a single member LLC, you want to be a multiple member LLC. The primary reason is to protect your company's assets from personal judgements. Or to put it another way, if you were ever sued in your personal life (say for hitting someone with your car), you want to avoid losing your company as part of any judgement against you.

It's kind of complicated if you are new to this sort of thing, but basically, if your company is a C-Corp, S-Corp, or Partnership, your ownership of the company is just like any other stock you may own in IBM or Apple. If you have a judgement against you, that stock is an asset, and the courts can seize it to pay off your creditors. More people lose their business this way than by having the business itself sued.

However, if your company is an LLC, the rules are different.

In an LLC, your creditors can not take ownership or control of the company. Instead, they only have the rights to any distributions that the company makes to its owners. To put it another way, your creditors can only seize the profits from the company when the company distributes those profits to the owners. Since you still own the company and make all the decisions, you get to decide when to distribute profits. And since you have a creditor with rights to those profits, you'd be foolish to do so. So instead, you'd hire yourself and pay out all the profits to yourself as a bonus, and the creditors get nothing. This is why an LLC is the best type of entity for a small, closely held business that is not looking for outside investors.

The reason that you want to be a multi-member LLC instead of a single member LLC is due to a technicality in the law. I won't go into the details, but there are some situations where a single member LLC is treated like a normal corporation and won't provide the asset protection that I described above. But mulit-member LLCs always provide that protection (as long as you incorporate in Delaware, Texas, or other states with similar LLC laws).

The good news is that even if you are a single person, you can create a multi-member LLC. Talk to your attorney about how to do it.

Do this now. In fact, here's the link.

As Justyn said, an EIN is a necessary step in creating a business. Remember, for you to enjoy to liability protections that an LLC offers, you need to treat the LLC as something completely separate from you. You are not the LLC, you manage the LLC.

An EIN and separate bank account are great first steps, but also be careful not to co-mingle finances. If you buy stuff for your LLC with your own money, then create an expense report and have the LLC write you a check. Etc. And so on.

Good notes above, but here's a few extra missing points:

1. Laws about LLCs and single-member LLCs in particular vary by state, so you might need to consult local advice, probably from an accountant.
2. No matter what, definitely get the EIN. It's silly to use your SSN on docs when it's so easy not to, and when/if you have employees you need an EIN anyway. Same with selling the company.