I think estimates of pitching dry yeast straight in turn up about 100 billion cells and rehydrating gives you 200 billion. So tossing it in would avoid overpitching but I don't think overpitching will really be an issue for you, like inhousebrew said.
To be honest I sanitize two glasses, rehydrate in one, then mix it up a bit, pour approximately half off, then toss out half and pitch half. Most likely absolutely unneccessary.
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