I’ll keep this simple.
Cheap (or free), wood is great. You save money. But ask yourself, is it worth your time.
Positive example: I got fence wood for free and only had to make a few adjustments for it to work with my haunt. Score one for me!
Not so positive example: I got some sheet wood from behind a flooring store (don’t worry, I asked). Barely any of it worked for me due to the composition of the wood.
Negative example: Free pallet wood but spent hours taking nails out of it.
Overall, I advise to manage what you need versus your time. In some of my projects, I have found that free wood (nails or nails free), works just fine. Other times I find it easier to just buy fence wood from Lowes or Home Depot. At $1-2 a piece it can save so much time compared to dealing with other wood.