In September 2010 the eBird developers wrote: "As we develop eBird, we're continually walking the line between building better tools that birders want to use, while maintaining our focus on collecting useful scientific data in the process."
All data submitted to eBird is valuable. However, when you combine your bird list and species numbers seen with effort--distance traveled and time spent--you make eBird data the most valuable it can be. These effort-based observations allow eBird to make the frequency and abundance charts that are such an informative part of the "Explore Data" function of eBird.
Estimate how far you walked to the best of your ability. Keep track of when you start and stop. Add that to your bird list and you've got it! If it isn't convenient to do it every time you submit a checklist to eBird, just do it when you can--every little bit makes eBird data that much better!
For more information read the eBird post: Effort-based observations enable powerful data analysis.