With rugby then generally longer is better. If you have a long lens (ie more than 400mm in 35mm terms) then you'd get decent results sitting behind the try line and having a go with that. Do not be afraid to go tight, there's no problem with shooting just the top half of players for example.
If you're restricted to shorter focal lengths then you'll be better off running the lines. I wouldn't recommend this to most people starting out but if you have a rugby background then you'll have a good idea where play is going to go and you can probably get away with it. Note that at the highest level people running the lines have a long lens as well.
You get very different images shooting from the end and shooting from the side as well, so ignore anything you read about finding a best spot and sticking to it.
And just in case Reportage wonders ( ), this advice comes from being the club photographer for the Newcastle Falcons since 2000, having shot at all the Six Nations stadia barring Rome, and having shot a Rugby World Cup. I have photographed players such as Dan Carter, Jonah Lomu, Jonny Wilkinson, Matt Burke, Carl Hayman, Richie McCaw, Shane Williams, Schalk Burger, etc. I also happen to have a pic of me with Jonny and a certain Webb Ellis trophy... shame I'm in it but can't do much about that
I do also happen to be a bit of a rugby fan, if you hadn't guessed.