My rule of thumb is don't use a focal length of less than 60mm. I learnt this the hard way.
Compare these images:
The lightning in the top four sections was actually closer to me than the big blue bolt on the picture at the bottom. On the third photo down from the top there are some small clouds in the bottom third of the righthand side of the picture, those clouds felt so close I could nearly touch them. The lightning was so close it scared me enough to make me pack up and leave yet the photos look nowhere near as impressive as the bottom one. This is because I used a wide angle lens (a 28mm lens - it makes things look further away). In doing so I broke one of my own rules - don't include extraneous crap - you can see around those pictures there is a lot of extra space, I should have used a 50-60mm lens and tightened up the composition.
Using zoom lenses is quite handy. That way you will be able to have more control over your composition and you don't need to bother changing lenses all the time. I use a 60 - 300mm zoom and find it quite handy. Unlike doing enlargements either conventionally or digitally, the quality drop off as you 'zoom in' with a zoom lens is virtually negligible. The only thing to worry about with zooming in is the effects of vibration. At longer focal lengths any vibrations are magnified, so the moral of the story is limit your zooming based on the wind.
Here are some examples of the use of zoom lens, the storm is about 60km away:
Zoomed in to 170mm
zoomed in to 250mm
zoomed in to 300mm
If this storm had been photographed at anything less than say 70mm it would have been that small in the frame it wouldn't have been worth taking