Is proof of accommodation REALLY required for a US citizen to enter Spain as a tourist?

A thought -- if you're staying at your boyfriend's home (or the home he grew up in) during your stay, wouldn't a "letter of invitation" from your boyfriend work? You could have him type something up, if it would make you feel better. I frankly think it's ridiculous that they would quibble with the fact that such a letter was not notarized by a Spanish police station. If they do, your boyfriend will be right there to say, "Yep, that letter is from me! I invited her to stay at my home in Spain!" Presumably the notarized thing is to confirm you didn't just invent a person and write the letter yourself. You could have it notarized in the US if you wanted to really cross your i's and dot your t's. It wouldn't hurt to make sure that nothing about your visit makes it seem like you are trying to immigrate to Spain. Because that's what they're really worried about, here. Pack light, make it very clear that this is a vacation. Have a confirmation for your return ticket printed out and ready to show them. If you have any appointments in the US after your return (dentist, work meetings, stuff like that) maybe print out emails to that effect. Be prepared to tell them all about how excited you are to visit the Prado or the Sagrada Familia or whatever.

Could you reserve a hotel and then cancel it, just to be safe?

Yep, me neither. I've never heard of this and I've entered many times.

You have nothing to worry about. Just write the address you're staying at on your immigration card and say you are visiting friends for Christmas. Please note that you're not applying for a Schengen visa per se, as you are covered under the Visa Waiver Programme for visits under 90 days.

I hate to be the one to say it, but here goes: Are you an inoffensive-looking white or East Asian person with middle-class clothes and luggage? Then you'll have no problem. These checks are to make sure a destitute foreigner doesn't come in and squat on their social welfare systems, and no customs official is going to raise a stink about someone who is clearly a tourist.

different consulates (and even officers within those consulates) will have varying levels of strictness To clarify, OP is a US Citizen and thus does not need a visa to enter the Schengen zone as a tourist for under 90 days.

I entered Spain twice, for stays of six weeks and three months. The former on a tourist visa, the latter on a student visa. Had a letter of invitation for the longer stay but was not asked to present it. No letter the second time. No problems. But this was ten years ago. Maybe things have changed, but I doubt it. In my experience, embassy warnings are always ridiculously conservative and entry requirements usually later than they are made out to be.

I flew into Barcelona in May, nobody asked to see anything but my passport.

You surely have nothing to worry about. However, as someone who has gotten worked up about the, at least, theoretical potential for something to go wrong at customs, here are two options that could assuage your fears: 1) Have your boyfriend's folks send you and him an email with their address, and saying how long you'll stay. Print it out. Yeah, it's not notarized by the Spanish Police, but I think you can quite credibly claim ignorance on that one. 2) If you're really worried about the notarization thing, book a room in a hostel for your first night and then cancel it. Hostel beds are usually like $25-30, and you only need to put a down payment when you book; you pay the full amount on arrival. The down payment is usually like 10% or so, so you'll only be out around $3-$5, and you'll have the peace of mind of knowing that there's really nothing that could go wrong at customs. That said, I know people who have overstayed visas in Spain. My impression is that their customs enforcement is very, very lax. It's not Spain, but when I went to Italy, the customs agent just stamped my passport and gave it back to me - he literally didn't ask me a thing.