"Am I being way too paranoid about this? :)"

Yes. None of this is the worry you may think it to be. Most chemical/synthetic ferts and pesticides breakdown very rapidly when exposed to sunlight, water and soil organisms. Any residual contamination will be very slight and is also unlikely to be taken up by the plant roots should they even reach far enough to come into contact with the undersoil. If you have any further concerns, just layer with newspaper. It is just newsprint (harmless and fully biodegradable) and printed with soy based inks.

The focus should really be on what YOU do to the garden and the methods that YOU follow, not what was done there previously.

Thank you! That does make me feel better :) It is our first go at this so any advice you may want to share about what we can do to keep the garden healthy is greatly appreciated!!

One further question. If we do not plan to use anything under the bed, is it necessary to remove the grass before piling all the new clean soil on top?

It depends on the type of grass - aggressive type lawn grasses, like St Augustine or Bermuda, will easily grow through quite a deep thickness of soil and are almost impossible to eradicate.

The grass around here is Quackgrass, " Elytrigia repens", "Agropyron repens", or "Elymus repens" which many consider a devils grass and I have buried that under soil and mulches with good results. This grass requires annual removal or it will take over a garden even if it is totally removed. Janet Macunovich wrote about putting in barriers to keep Quackgrass out of garden beds, barriers buried 18 inches deep, vertically. Some grasses will always be a problem no matter what one does.

Ashley - What kind of grass do you have? (where do you live and we can guess)

Some of the warm-climate grasses, Bermuda and St. Augustine) would not be slowed down by being buried and would send up runners to take over the bed.

The more delicate northern lawn grasses will die if you bury them deeply.

We live in northern Illinois, just barely south of the Wisconsin border. I'm not sure of the grass types, but I do know that we have one broad leaf grass/weed that sends really long (upwards of 2 feet) runners into my flower bed in the front yard. However, that bed isn't raised and doesn't even have a border. We also have a lot of clover. The location of the raised beds is going to be over a section in the back yard with few weeds, just lawn grass mostly. I plan to make them fairly deep...A foot or more.

The grass you describe, Ashley, sounds like Quack grass. You could take a sample of the grass to your local office of the University of Illinois Cooperative Extension Service for positive identification.

http://web.extension.illinois.edu/state/findoffice.cfm

Thank you for your help and thoughts to consider, kimmq!