To my knowledge, the only reason you take folic acid is because long-term MTX use can lead to folic acid deficiencies (which can cause side effects such as mouth sores). Whether you're taking folic acid every day, 48 hours after taking your MTX dosage or once a week shouldn't matter, as long as you're taking the prescribed amount. I don't think taking the folic acid immediately before or after your MTX dose would have any impact on side effects such as fatigue, muscle weakness, or nausea.
I take folic acid every day and have since the week before I started taking the MTX and I still experience side effects every time I take my dose. It's not nearly as bad now as it was the first few times. The most important thing when you're taking your MTX is to stay hydrated. I take mine immediately before bed Friday night to avoid the nausea and so I don't have to worry about missing work if I am really worn down the next morning.
29f, PhD student, originally diagnosed with Crohn's in 2/06
10/09: Prometheus test shows results consistent with UC
10/12/09: beginning treatment for inflammatory arthritis
Currently taking: 7.5mg methotrexate weekly (pills), 2g Pentasa, tapering from 20mg prednisone, 150mg Trazadone, multivitamin, vitamin D, magnesium, probiotics, folic acid