Using a speaker on its side as your center channel

[househead]

OK, here's the situation:

I am considering going with 5 bookshelves for HT (currently have 2). Is laying one on its side and using it as a center channel a *huge* no-no? Or will I barely notice a difference VS. having it straight up? It's a traditional 2-way dome tweeter/mid-woofer design. Would it cause any major sound dispersion issues?

There isn't a center channel in the line and I've got no space for it standing up. Should I scrap the idea and go phantom center?

Any input is appreciated!

Thanks