Just keep it the same. You can use all the slang you want - but if you copyright something spelled a certain way, put it on your album that way, and register it with BMI/ASCAP that way. Sounds like a no brainer, but you would be surprised how many songwriters don't do this.
This isn't a legal thing, it just makes songs easier to identify if they are reported. It makes my life easier as well.
Also, if you want, the PA form has an alternate title field.
So do song registration forms with ASCAP/BMI.
So if you record the song "I gotta get movin' B4 da train come in"
you can also put "I've got to get moving before the train comes in" as an alternate title on both the copyright form and the performing rights society's form. Just increases your odds at getting royalties if the song gets performed. People who fill out cue sheets for radio/TV sometime use poetic license when writing down song titles.
Brad