Does your tuner have a long history of reliable tunes, good economy, and drivability with the A/F readings from their equipment? If your tuner always uses one brand/type of wideband and one method of placement for measuring air/fuel, let your tuner continue to use his preferred device and technique. If your relationship with the tuner is brand new and you feel the results are less than perfect, ask around for feedback from other enthusiasts and change tuners.
Here is some interesting information regarding AEM wideband accuracy:
Several major automakers have used AEM wideband units during engine, performance part, and racing development programs. Mopar Performance engineers for instance, used AEM widebands during the development of their 5.7L Hemi, 6.1L Hemi, and 392 Hemi crate engine.
Last year, a Chrysler engineer responsible for some 6.1L Hemi long-term durability testing reported back to us about some really interesting data. During a 200 hour long durability test, a pair of our inexpensive little AEM wideband sensors were mounted in the same collectors as a pair of laboratory-grade $5,000 Horiba controller sensors. He found that the logged readings from each brand fell within one to two tenths of each other throughout the range of the test (10.5:1 A/F gasoline to 15.5:1 A/F gasoline).