If you're going to shut down an engine in flight, regardless of how you accomplish it, be prepared to not get it back. Regardless of weather you're 3,000', or 3'. Unless you're fully prepared to deal with that consequence, then don't do it. Period.
Killing an engine with the fuel selector is fine and dandy, except that you're starving your fuel-wetted/lubricated components, primarily the engine driven fuel pump. That increases wear, and also the possibility of a failure. Not just at the time you're simulating the engine failure, but later.
Stopping fuel flow introduces air throughout the fuel system, leading to an increased possibility of hydraulic lock, as well as the near-certain liklihood of pump cavitation.
Killing the engine isn't necessarily a bad thing; if one is prepared to land the airplane it doesn't matter how high it's done. During my ATP ride, the examiner killed the engine at 200'. I had made it clear that if the engine quit below 400', we'd consider it an emergency and treat it as such, and I did. Unable to mount much of an effective climb, and unwilling to seek a restart there, I returned to land on an intersecting runway, with a very low pattern. I'm not going to second guess the examiner in that case; he was outside of the Administrator's guidelines...however, we were continuously in a position to make a safe landing, and we did. He also pulled the engine using the throttle.
He also knew that I've spent a great deal of my flying career below 200', and that it was a comfortable environment for me. I don't know if he would have done that to someone else, or not.
I don't have a lot of seneca time. About four hundred fifty hours of so. The Seneca II or III will easily hold 8,000 MSL with one engine out and feathered, and the airplane full of fuel and people. It's got quite a high single engine service ceiling. The Seneca I doesn't have that high a service ceiling, being normally aspirated, but it's still high enough, so long as one engine is feathered.
Messing with fuel selectors during taxi is not a very wise thing to do. Releasing debris that may be trapped behind selectors, or having a selector fail without your knowledge in an intermediate position or an off position, can get you killed. You may have just enough fuel between that failed selector and the engine to keep it running through the takeoff...and then you lose it. It happens. Debris that may be trapped behind the selector valve can be released, just in time to mess with a fuel controller, injector plug, carburetor jet, etc. Just not a wise idea...because it's quite likely to get to the worst possible place, at just the worst possible time. Leave those selectors alone. Either check them before start on the preflight before a long taxi and runup, or don't touch them until you're over a safe landing site once airborne.
Block rudders for engine-out work and Vmc work, stick with zero thrust for most single engine work (using a throttle), and always warm up an idled or killed engine before going again. For those of you conducting single engine approaches, no go-arounds. Land, warm that engine back up, and go again. If you've shut one down, warm it up.
On the subject of going around single engine, I once had an examiner demand to see a single engine go-around in a Seneca II. He told me I would be flying a single engine ILS under the hood, then executing a single engine go around. I told him I wouldn't. The checkride was for a small 135 operation. He told me I'd not pass, I told him I didn't care. He told me it was a test standard, I told him it wasn't. Not unless it's a part 25 airplane, and the Seneca II definitely isn't. He got lippy, told me I was goign to fail the ride. I told him that would be fine, and asked him if he preferred to get out now, or wait until we got back to the airport. He shut up and finished the ride, I passed.
He asked what I would do if I flew a single engine approach in the airplane and found the runway fouled on arrival. I told him I'd land anyway. But what if the runway is blocked? I'd land adjacent to it. Wassamatter, Mr. examiner? Never landed on grass? In snow? On anything but hard dry pavement?
I don't know that I'd agree that going around will kill you...but it won't do you any good, and your odds aren't great of success. Especially at any significant density altitude of note.
