I disagree with that last, often times even if there is a warrant LE knocks and asks for consent.
I'd ask them what they were looking for. They will tell you something specific, find out and maybe show them.
It never hurts to give them a pleasant experience and to be seen as helpful rather than uncooperative.
For example, the ATFE agents say they want to see the three ARs you bought last month at the gun show, to make sure you're not reselling.
Thought experiment 1:
"Sure, one is in the car trunk, I'll show you that one first. Two of them are in the safe, I'll go get them- my house is a mess, just wait here on the porch. I'll break them open so you know you're safe. Would you like a cup of coffee?"
ATFE agents look at guns, leave pleased that you made their day easier, and figure that you are a solid citizen. It takes ten minutes, and all they see is those guns.
Thought experiment 2:
"You got a warrant?"
2A- "No."
"Then go away. "
Agents leave. They have to report their own failure. They figure you have something to hide. At best, you go into the jackass/probable criminal/probably dangerous category. At worst, they decide to investigate you more closely.
2B:
"Yes, we do. "
You get handcuffed and sit in a very hot car for three or four hours while the agents see EVERYTHING YOU OWN, cut into your gun safe, look at your computer, etc, etc. Assuming you aren't charged, you STILL go into the jackass/probable criminal/probably dangerous category, as well as having all the suspicious but not criminal stuff they saw listed. Your "arsenal", your "anti government web sites", your "thousands of rounds of ammunition".
With a bonus listing as "We just missed that guy, next time..."
And they WILL investigate you more closely.
ATFE is law enforcement, aren't we in favour of that?