Homeskin

What country are you from?
I'm telling you, you can save yourself a headache by just claiming FTC (unless you are from a no-tax or low-tax country).

Sometimes it's more advantageous to just claim FTC (Foreign Tax Credit).
Income Tax in the UK (for individuals) are generally higher than here.

UPDATE: 
 
Hi guys, 
 
I thought I'd post exactly what happened in case it's useful to anyone in the future - to cut a long story short my application was successful. Nice.
 
On the affidavit of support I went prepared with two filled out: one was my fiance and the other was her Dad. 
In the end they took both affidavits to review (they collect your documents in the first instance and then take them round back). 
 
For each I had the filled out I-134 form and tax returns. I also had a letter of employment for my fiance which they didn't want to review (despite my asking). They took all of the forms to their back office (which is normal practice) and everything was fine - they only request they had was proof that my fiance's Dad and Mum (both were on the tax return) were US citizens (like a passport photocopy).
 
I said I didn't but he said it wasn't a problem and everything got processed as normal. I assume this was to find them more easily on their system or something and the tax return was probably enough. 
 
Conclusion:
I went in there unclear as to whether my fiance's contractor job status and only having worked for 3 months (having been studying before) counted against her despite having a very healthy salary. Unfortunately, I am still none the wiser. 
 
I decided to take a second affidavit (which, in the end, wasn't that difficult as my fiance's Dad's salary was over the poverty limit so all it needed was a completed I-134 form and a copy of his tax return- didn't need proof of assets). My advice is that if you're in any doubt, get hold of these - it's totally worth it for the peace of mind and it makes life much simpler.