I personally think you are wiser to put down 30-50% as a protection against market fluctuations.

Remember rents can fluctuate.

However, I haven't been able to comprehend how a long term buy/hold investor would be significantly negatively impacted by fluctuations in the market...assuming of course that you have a property with a tenant that passed your screening criteria, you adequately prepared cash reserves for repairs/maintenance and cap ex, bought your property with a fixed mortgage, and essentially took the processes advocated for here on BP, I don't see how a long term buy/hold investor for rental properties would get burned?

The market fluctuates but fundamental changes are fairly rare when you look at the big picture. 

Had a plumber come out and he said he feels a slight fluctuation in the water when he has the bathroom faucet also running and he's flushing to toilet.

@Adam Juodis it is good to have an "idea" of your net worth even though owning real property makes it difficult to have an exact value ( properties values fluctuate). 

Because I am in and out so quickly, large fluctuations are not prevelent.I would go further, but I fractured my wrist this weekend and it is hard to type with one hand.

For a mortgage it is making sure you have a long time low interest debt service locked in.Materials is having fixed pricing contracts where you do not have wild fluctuations in your wholesale cost.

If I buy a 100K house that appreciates 3% over the long run (per study by famed economist who called the last crash, Robert Shiller), in 30 years I have ($) 242,726.25 Jack - In this example, you are picking a period that includes many market fluctuations - ie a few recessions, etc.

It'll hover there and fluctuate as supply and demand dictate; as well as technical advancements that make shale more cost effective.