When importing products, do I have to fill out the income and expense acct fields?
QuickBooks Online has a few options when it comes to adding products to your books in the program. Entering them manually is one way, and it sounds like you're using the other option of importing the products through an Excel sheet. This is handy when you've been keeping track of your inventory information outside of the program and allows you to easily import many items at once. I'll go over this with you so you have an idea of what steps you need to take.
When you're importing from Excel, the QuickBooks Online will try to map the information you've given to the fields it's used to with it's built-in features. If you leave fields blank, the Map Data stage of the import will simple read "no match" and you can either choose information from there or you have the chance to make some change on the next page where you can review details as well. If you don't choose in the Map Data stage, QuickBooks will use default accounts in the next area.
The income and expense accounts are important details with your products as this will help QuickBooks know where to allocate various funds and information for your books. I've got a few articles below that should help with getting a handle on the import process and QuickBooks Online's inventory system in general.