Printing through negative sleeves is a little like keeping a UV haze filter on your lens for protection, and there are many long threads on the pros and cons of that. FWIW, my preference is to print through the sleeve and not use a haze filter.
As stated already, keeping the negs in the sleeve means there is less danger for scratches in handling, the downsides are that the proofs are slightly soft, and the sleeve adds a little density that you will need to compensate for with exposures.

Assuming you are using thin sleeves meant for contacting; if your sandwich of sleeve, negative, paper and glass is tight, the resulting contacts will be soft, but they should not be soft to the point of being unusable.
If the prints look unusably soft and the glass is heavy enough to press the negs and paper flat, then it's likely the negatives really are soft.

My practice is to look at the neg, with a loupe if necessary, to judge sharpness, and use the contact to judge exposure and composition.