If it was an odd shaped room and buyers would benefit from a vision of how it could be used. 

Any room that is difficult to imagine furnished, needs professional staging!

I don't stage properties... sellers do. If it wasn't selling vacant, I'd make a strong recommendation... but a lot depends on whether seller is willing to spend the money. It's not cheap to stage.

good staging always helps. If the seller won't or can't pay for professional staging, then we add some small touches here and there depending how much stuff I have left in my house....

When it is one that I would like to get more money from, the level of staging would depend on the price category and owner interest of the property.

I let sellers decide, most are not willing to pay for it, though. 

Just like Inna Ivchenko said, always unless it is a fixer upper in rough shape.  Every house can use a tad of warming up with staging.

I believe all homes (vacant or not) could benefit from staging.  It's a matter of whether the seller is willing to invest.

If the floorplan was hard to figure out on it's own or if people thought the bedrooms were too small.

Staging will bring a higher price.

If I think it's necessary, I would recommend it. 

I mostly sell vacant bank owned homes over vacant re sale homes. Therefore a simple staging wont do anything for most bank owned homes that need a hammer or a bulldozer

If the seller is willing.

Susie Kay nailed this one!

  Anything over $200K can use some staging.  Will the Seller pay?

If it is over 500,000 in our area I would.

Most, unless it is a fixer or similar condition priced accordingly.

I was taught by some lady builders to create small vignettes in a vacant home to add a little bit of color and pique the imagination of a prospective buyer. It makes a difference.

Most any vacant property, minimal at least, if at all possible. Need to convince the owner to open the wallet here.

If it is higher-end and sits on the market vacant for too long.  

If properties in the area were not selling in the first couple of weeks. 

A  or upscale home

100% of the time UNLESS it is a fixer or tear down. 

when we have a vacant home, we have our photographer do virtual staging for us....and we enlarge the photos and put one in each room on the first floor on an easel.... 

It depends on the property.  If it is a contractor's special, then it will not need staging. A

Under what conditions?

Priced at the community ceiling.

Has too many unassigned spaces

Needs props to engage buyer into lifestyle change.

Blemish obvious such as too many floor types viewable from one location.

Stager offer a deal I can't say no to.

To photograph it

I do believe vacant properties can definitely benefit from staging. It helps buyers see the size of rooms and possible uses much better. 

I recommend, I don't stage

Steve Penner - if priced higher than perceived market value, I'd ask the seller to stage the vacant home.

When my client agrees to the cost...

Difficult floorplan.

IF it hasn't sold in a reasonable time...house has irregular shaped rooms or odd configuration.

If sellers want ti pay for staging by all means. That includes condos as well. The price is close to $3K now. Still no inflation?

Years ago, in a declining market, a new vacant home wasn't selling. Then, they staged it and sold within a week. I saw the before and after and was impressed. Market, product and top dollar all play a part in considering this

Pretty much all the time.

A furnished house vs an empty house, I personally don't think it makes a difference.

Depends.