Believe it or not, the "drafting" effect and bow wave are more of a factor than wake turbulence. Like B747Skipper said, if the closure is performed correctly, the receiver will not directly enter any wakes, but as the acft become closer, a drafting effect similar to a car behind a large truck on the highway comes into play. On smaller acft, such as fighters, the bow wave is small and the drafting only effects the receiver throttle responses, but in larger acft it is much more pronounced. As a large acft approaches, they must first break through the layer of air being created by the bow wave of their acft and the fuselage wake of the tanker. As this occurs, the closure rate is approx. 1ft/sec, the autopilot will trim out the nose down effect on the tanker, and the tanker pilot actually slowly retards the throttle to counteract the pushing effect of the receiver. The receiver acft then becomes extremely pitch sensitive and must react very carefully to prevent over-controlling the acft. When the two acft are in position, the overall aerodynamics are as if they are one aircraft, and thus the motion of one directly affects the other, causing an increase in overall airspeed and a tendency to increase closure speed due to the Bernoulli affect between the two acft.
Yes there are times when Air Refueling cannot be accomplished due to turbulence, but it is due to natural turbulence, and not acft induced.
The other factor is the weight change between the two acft. If the tanker is heavy, and the receiver is light, at the end of an AR, it is reversed, the tanker is light, and the receiver is heavy. Due to this, the performance capabilities change, and constant small corrections must be made to facilitate for this. The receiver always flies AP/AT off, and the tanker normally flies AP on and AT off, with the PF keeping one hand on the throttle and one on the yoke at all times. Practice is required for both acft to fly AP/AT off, and is much more involved.
Hope this helps,
"Never trust a clean Crew Chief"