In capitalizing English titles, my understanding is that all prepositions of four or fewer letters should be written lowercase, unless part of a phrasal verb. (I realize that AP style and the Chicago Manual of Style differ on the length of words affected, but I'm going with the "four or fewer letters" standard.) For the most part, following this guideline is fairly straightforward. However there are certain rarely used prepositions (or words that rarely function as prepositions), which in practice I almost always see capitalized even though they should (in theory anyway) be written lowercase.
This is especially true of "down" which always seems to be capitalized, even in publications and on websites (like Wikipedia) that normally don't capitalize prepositions of four or fewer letters. I'm not sure if this is a commonly accepted practice (like the way "as" is frequently written lowercase even when functioning as a subordinating conjunction) or if it's just a result of confusion because "down" is so rarely used as a preposition. There are a few other words which I sporadically see written lowercase, though not consistently.
I've listed a few examples below. All spellings are taken from Wikipedia. Any suggestions on how to capitalize these would be very much appreciated.
All Down the Line - "down" accented/emphasized; capitalize?
Backs Turned Looking Down the Path
Blowin' Down the Road
Buddy Won't You Roll Down the Line
Goin' Down the Road Feeling Bad - phrasal verb? (as in "Going Down to Mexico")
Hasten Down the Wind
Kick the Bride Down the Aisle
Ninety Miles an Hour (Down a Dead End Street)
Walk Straight Down the Middle
Way Down the Old Plank Road - preposition?
When My Boy Walks Down the Street
Whistle Down the Wind
Climbing Up the Walls
Farther Up the Road
Going Up the Country - "to" implied? ("Going Up to the Country")
Halfway Up the Stairs
When the Roll Is Called Up Yonder
Running Up That Hill
On Down / On Up:
Bring It on Up
Farther on Down the Road (You Will Accompany Me) - never see this written as "Further On down the Road"
Further On (Up the Road)
Move Me on Down the Line
Movin' On Up
Dirt Off Your Shoulder
Falling Off the Face of the Earth
Get Off the Phone
Give Up the Funk (Tear the Roof off the Sucker)
Get Your Filthy Hands Off My Desert
I Came as a Rat (Long Walk off a Short Dock)
Keep Your Hands Off Her
Knocks Me Off My Feet
Take a Load Off Your Feet
Your Phone's Off the Hook, but You're Not - "Off the Hook" used idiomatically; capitalize?
Already Over Me
Cry Over Me - phrasal verb?
I Don't Want to Get Over You - phrasal verb?
I'm Getting Sentimental Over You
If I Had Possession Over Judgment Day
Look Over Yonders Wall
Meeting Over Yonder
Someone to Watch Over Me - phrasal verb?
Somewhere Over the Rainbow
(subordinate conjunction - always capitalize?)
Do Nothing till You Hear from Me
Good Til Now
I Hadn't Anyone Till You
Love Me Till the Sun Shines
Love You till Tuesday
Rub 'til It Bleeds
Tango Till They're Sore
Wait Till the Summer Comes Along
Wait till You See Her
You Don't Miss Your Water ('Til Your Well Runs Dry)
'bout: (What's So Funny 'Bout) Peace, Love & Understanding
'cept: Nobody 'Cept You
out: Goin' Out West - preposition?
past: Whistlin' Past the Graveyard
unto: Do Right to Me Baby (Do Unto Others)
upon: Once Upon a Time - "Once Upon" an idiom; capitalize?
upon: We Call Upon the Author - phrasal verb?
yet: Seasick, Yet Still Docked - coordinate conjunction, but never see lowercase
By the way, these are all song titles. Not sure if the standards are different from, say, headlines or book titles. Thanks for any help with this.