It was discussed a while back, quite a lengthy dissection of it all as I remember...

We all learnt something from it though, if you fancy a good read through here are the two threads:
http://www.theguineapigforum.co.uk/showthread.php?t=40044
http://www.theguineapigforum.co.uk/showthread.php?t=40177
The basic idea is that regular treatment with Ivomec and Panacur does not rule out the chances of parasitic infection, it only treats what is present in the system at that time, but because the treatment is regular it regularly eradicates the parasites, usually preventing them from affecting the pig too severely. There is a particular parasite Vedra dubs the "silent killer" because it burrows deeper down and does not cause any symptoms until weight loss is noted and a degree of damage is done internally. Treating with Ivomec and Panacur once every three months reduces the chance of this parasite reaching this stage.
In particular, hay quality plays a big part in whether worming is going to prevent parasitic infestations or not. There isn't a great deal of use in worming a guinea pig if the pig goes straight back to eating the very hay which could have caused a parasitic problem. It kind of defeats the prevention side of the worming! Thus a hay that is known to be free from parasites should be fed. Oxbow is said to be the best as it is imported, thus quality- controlled - the authorities won't allow products likely to be infested with parasites enter the country. Alfalfa King is along the same lines, imported and good quality. UK-grown hay
may be more likely to contain parasites or fungal spores due to the climate in which it is grown and harvested; the dampness of many bales are ideal homes for such nasties.
At the end of the day, no hay is going to be 100% guaranteed parasite-free, but it's seriously worth considering the highest quality you can afford.
It is only in the past year I have treated my pigs routinely. I was pretty much convinced after Jasmine suffered unexplained weight loss. No other symptoms, but she was consistently losing 1-2oz a week for a short time. Out of interest I gave her a one-off dose of Panacur and the weight loss stopped straight away, she regained the weight. Since dosing the pigs regularly, there have been no cases of such unexplained weight loss.
No idea on how the Panacur Rabbit can be used, alcesterpigs mentioned that he may try to find out. At the moment, the recommended product is the 2.5% sheep worming solution. The Ivomec is either the cattle injection fluid, or the Xeno 450 topical treatment (since the Xeno is said to treat both internal and external parasites).