Just to be a little bit more precise, that is the LAST sheeting you will glue that will lock everithing in place. I am building the exact same plane right now and I also had a slighly warped wing and everything is OK now after the bottom sheeting was glued about 30 minutes ago
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If the wing is still warped after the sheeting is in place, I think it is better to have the wingtip with some washout than some washin, it will help prevent tip stalling.
I hope this helps.