I definitely prefer early morning or late evening. Last year I had great success with blues just after sunrise (around 6am). Cold and inactive, often covered in dew and with superb bright lighting. If you have a good site with lots of shady areas, you can have several hours of this as the sun hits different parts of the site as it rises above the trees, hedgerows etc.
Grey days can be good any time of day also as the butterflies are colder and less active.
Another great one is partially cloudy days. When the sun is out they are all over the place but often freeze as soon as the sun goes behind a cloud. I recall my first orange tip (after many fruitless days chasing them!) was photographed this way. I just followed one for a few minutes until a cloud came over and it settled almost instantly and closed it's wings. It was then so docile I could pick the thing up without it attempting to fly off. Got a few shots and then within 10s of the sun coming back off it was off again.
Basically any insect is easier to photograph when it's cold. I use very similar methods for dragonflies too.
I have also found that for some species when it is extremely hot they can also slow down. Didn't get much chance to test that theory here last summer! Not sure if others have noticed this?
Also, the habits of some species change through the day. Middle of the day seems to be pairing up day for some species, blues especially. Egg laying time for others. These are also situations where the butterflies are a little easier to photograph. Pairing up especially so.
A Brown Argus taken at 7am last summer:
![Image](http://myweb.tiscali.co.uk/johnbogle/BC/_MG_467222.jpg)