One Domain Registrar Pro's:

1.  Easy management - obviously only have to go to one place manage your domain names.

2.  Bulk Pricing Power - Usually get discounts for bulk purchases.

3.  Learning Curve - only have to learn one system so it's quicker.

One Domain Registrar Con's:

1.  Disappearing Registrar - If they disappear you may not get your domain names back for awhile and possibly forever.

2.  No Choice - If you find something really annoying about the registrar you are stuck with them unless you take the time and expense to move all your domain names.

3.  Sales - If you stay with one domain registrar you won't be able to take advantage of sales from other domain registrars.

Multiple Registrars Pro's:

1. Choice - you can have some domain names on your highly secured registrar and others on the domain registrar with better prices.

2.  Sales - Chance to take advantage of sales offers from different domain registrars.

3.  Diversified Risk - Multiple registrars lessen the risk to your domain portfolio if something goes wrong with the domain registrar.

Multiple Registrars Con's:

1.  Domain Management Nightmare - You have to track which domains are on which domain registrar and keep track of renewals and any correspondence.

2.  Pricing -  Other than sales specials you will probably get better pricing buying in bulk instead of spreading out your domain name purchases.

3.  Learning Curve - You will have to learn to navigate multiple domain registrar interfaces which will take time.  On the other hand you will also learn a lot along the way.

At the end of the day it's up to you.  Personally we recommend that you use just one domain registrar and as you grow into domaining then you can think about spreading your domains out if you really want to.