I've been reading a little on this the past few days and the general consensus is yes. It needs to be under control for several reasons:
-difficulty concieving (irregular cycles, not ovulating)
-higher risk of miscarriage
-risk of birth defects.
So, yes it's important! The good news is that it seems when properly treated conception and pregnancy can proceed without significant problems. Treatment and monitoring are throughout pregnancy are also crucial because levels often change during pregnancy.
I just got a book from the library by Mary Shomon. She has several books and some articles online. She goes into detail explaining hypotyhroidism and she's not hard to read. Do you know your TSH levels? If not, try to find out because the optimal level (especially if you are TTC) is 2 or lower. Some doctors think 5 or 6 and lower is fine but the new recommendations are 2 or less!
Good luck, I hope this helps.