lutrinae you do what suits you.
I had originally planned 2 weeks mat leave last time, but made it earlier in the end as I found I was getting really tired. My body was telling me loudly to slow down! There's absolutely no way I could have gone to work the last week. I had to ask my sister to come round to mine to help me sort through stuff, it was beyond me - I was having problems standing up for any length of time!
A good friend of mine on the other hand had loads of energy throughout her pregnancy, and she worked right up till just a week before her EDD.
I would say though, you do need a back-up plan if you find you're not coping. Don't ignore to your body if it's telling you it needs to rest! (And this is a real possibility.) You need your strength for labour and the first few weeks.
Would your work be flexible if you needed to take more time off at short notice?
Slightly tangental, but have you got a mortgage? If so, do you know if they give mortgage holidays for mat leave? Ours did and this allowed me to take a year off - would have been 6 months otherwise.