I dont know what you mean by " saddle ".
To avoid showing how thick I am, I have been googling about saddles, amazing how many different saddle makers etc.
Then had a brainwave and googled geology-saddles, ended up in the ocean, and now know that saddles dont occur deeper than 200 mtrs.
So here I am showing how thick I am.
Strange thing is I went on a barrow hunt yesterday, found some as well.
I find a church, from it in a cross pattern, N,S,E,W , I wander along as you know I do, and hey presto within usually several hundred yards, bumps.
Round bumps is where I find circles, long bumps is where I find spirals.
The churchs have the best spots, usually both a circle and spirals, west kennet long barrow has this setup.
I would not build a church , down at the bottom of a valley if it were on sandy ground and liable to flood, and sink, and you dont see many graveyards where the coffins would flood and sail away, do you.
I know you dont talk about churchs on here, or you think you dont , but if you were to stop and consider that the long barrows were a construction built for the same purpose , at the same spot , as churchs, well on the whole we dont bury anyone in our churchs do we?, they used to put the leaders in the biggest ones, but mere mortals are plonked around about.
Kevin

Can't say that saddles are popular 'round my way, but as Tiompan mentioned tracks do tend to use saddles as they tend to be the easiest upland crossing points and of course many barrow groups are often associated with trackways. Another thing to possibly consider is that some barrow groups are thought to mark prehistoric boundaries, I guess a saddle is a good place to for a boundary.
I guess the things I'd be looking for would be visibility and intervisibility with other monuments or areas of occupation.
Although not a barrow, this site is tucked nicely into a saddle http://www.themodernantiquarian.com/site/2609