Occasionally, I need to take care of an issue, but mostly, it's handled by my contractors.

You MUST be willing to occasionally travel to your properties or send someone who is deeply invested in the property like you are - not your own PM.

In that case while they were initially apprehensive all the tenants agreed to sign new MtM rental agreements (I told them I was not re-screening anybody, they would remain MtM,  and I laid out clearly what rent increases were coming the next 2 years)It sounds like you have been at least tangentially aware/involved in operating rentals for a while,  but if you are at all uncomfortable with the prospect of managing tenants and the occasional need to be firm or make hard decisions,  hire a property management firm.   

I know a couple that has a condo there they use as an occasional weekend place and do shorter term rentals. 

Occasionally clients will pass angry mail recipients off to me as "the source".

Or would you use these properties as vacation homes occasionally? 

Mostly because the net income by then will be around $500 or $600 per month per house which is more than enough to handle the occasional big ticket repair.The one thing I will say I understand is that I wouldn't want to have to count on my real estate income to replace my job income at a 1 to 1 basis.

Occasionally, they don't even have it then!

Occasionally you will run across a listing that lists actual numbers.

I agree with Anson that a multi-unit would be great, and occasionally I see one that works for an owner occupier, but you could probably do just as well with a couple condos.