Why do deviated wells show poor correlation to seismic data?
Two-way time to depth calibration is a vertical problem.

How you handle deviated wells probably depends a bit on how you are tying the wells. Here are two things to watch out for:


You should be tying to true vertical depth (TVD) anyway — make sure you're not using measured depth somehow. I expect you are using TVD — so the deviated section will be distorted (squished). It's gong to be difficult to tie, and the modeled waveform won't make much sense.
Sometimes software tries to help you by placing wells at the midpoint between the top- and bottom-hole locations. You don't want this — make sure the well is at the top-hole location, or at the midpoint between the top-hole and kick-off point.


Because of these two things, you may want to ignore deviated wells in the tying effort, if that's an option. If it isn't, focus on tying the straight hole.

Ultimately, you'll want to build a 3D velocity model, using horizons to interpolate the velocity field, and maybe even using other velocities (like stacking velocities) to 'fill in' where there are few wells. At that point you can position everything properly, deviated or not.