The series I read mostly aren't available in e-book.
But the few which I really love enough to buy for myself, I've been picking up the e-books if they exist when I can catch them at a good low price (generally $3-5 after discount) and still buying a paper version to support the author/local used bookstore, depending on what's still in print.
For the record, I have no problems with the transition between paper and e-reader and do it quite often, but other posters have noted that their mileage varies. And I'll read "my" books in whatever format's available to me at the price I'm willing to pay (which includes free from the library if the writing quality has been deteriorating).
I say if you're not reading some of them right now and not collecting in a time-limited format like hardcover, let those lapse until you start or there's some sort of sale which seriously drops the price. Otherwise you can always go pick up the paperbacks/e-books at your leisure.
For the series you read currently, it's up to you whether you think you'll prefer e- or paper versions. If you get sick of them, you can always sell or donate the paper books.
On the other hand, e-books take less space and may be cheaper and you can keep them around for "I can't believe I used to like this!" value for years without cluttering up your place or worse, going on a mistakenly nostalgic hunt to buy and try them again.