The majority of engineers will want dry stems from you so they can control the effects; after all, that is what you are hiring them to do.  A dry stem simply means remove all effects.  Some samples might come with an effect baked in if it's from a 3rd party.  Just leave these however they came to you. 

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If you have tuned your vocals and like how they sound, leave the tuning on but turn off any EQ, Compression, Reverb, Delays and all other effects. 

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If you have a "special" sound effect you like send a dry and a wet version.

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Bottom line is at minimum, send dry stems but send wet stems IN ADDITION if possible.