Former cop here: You're right on the first part.
Until you realize your friend left his gym bag with weed in it in your trunk. That looks bad man, and comes back on you. My best seizures came from consent searches. I've also had cars that I've stopped where I was darn sure they were moving something in it, but I didnt have PC, and I didn't have a dog nearby that I could get in reasonable time. I asked for consent, and got told "No" in no uncertain terms. They left on their merry way, and I still wonder sometimes if a compartment full of stolen guns was in that dang car.
Sometimes we have a reason to be suspicious, sometimes were just guessing. Just to account for confirmation bias, I'd ask for consent to search completely randomly. Sometimes I got it, spent 10 seconds searching, and was done. Sometimes I didn't and said have a nice day. Sometimes I got the consent and a stolen gun and some hard narcotics
There have been a few cases where we had enough PC to search a residence, and asked for consent. Each time it was an unusual circumstance (e.g. the roomate was selling drugs or wanted or something). Each time we were in and out in 10 minutes. If we had been told to get a warrant, we would've had to detain the home owner (which in this case means hang out with him in our car or in his yard, casually talking and killing time) while we go to get a warrant.
This means driving to the magistrates office 45 minutes away, writing the affidavit, waiting in line, doing the hearing, then driving back, maybe 2-3 hours total. If during that time the homeowner goes "Hey, screw this, just search it", we won't, as we don't want it to seem like coercion. Once consent is denied, we aren't going forward without a warrant.
So again, general rule, "Dont talk to the police, assert your rights". Except when maybe you should. If you come home and your spouse is missing and blood is everywhere, you are a suspect. Heck, at the beginning, you are probably THE suspect, but you may want to talk to the cops to help them figure out where your wife went. If the cops want to look in your house for a stolen gun that your shady roommate allegedly stole, it may be BS to just search your house, or they may just want to check that 1 room, get that stolen gun, and go.