I don't know if I have too low standards for old bills but my minimum grade requirements for my collection goes as follows:
Large notes: F, F/VF, nothing above except 25 cent notes
1935 $5, $10, $20: Between F and VF <--- too low?
1935 $2: VF+
1935 $1: EF
1937's: At least one EF/AU in each (VF okay for $50 and above)
1954's: At least one UNC for each, EF/AU okay for $20 and above.
*Majority of non-UNC notes obtained at face value, including some low grade.Multi-color: All AU/UNC (at least a few UNC until $10 denom), lots of EF and below (obtained at face value)
Birds: UNC, consecutives, from the bank, AU okay for $100 and $1000 (all obtained at face or little above face)
*Collection ends here. Nothing newer than Bn-Th, nothing to be added, except Cr-Bo.Journey: UNC, straight from the bank, only low denom's so far
As you can see, I have pretty much completed my collection. I now buy and sell but more so, focus on the very rare notes that could single-handedly boost my collection, like a pre-1900 in nice condition.