This is dependent upon how likely you are to publish a new training manual when the feature leaves beta.
If you are regularly updating your training materials, so that any changes to the beta feature come with an updated manual, and your users use the updated manuals, then there is little harm in pointing users towards features that are helpful, provided that it is made extremely clear that the feature is currently in beta.
If it is likely that updated manuals won't be made available to your users it's best to keep references to features that are likely to change out of the training literature until they have been finalized.
If the feature is extremely beneficial you can always point to it at the end of the manual, along with links to more thorough documentation, as additional reading.
If the beta feature is complex enough to require dedicated instruction then it might be a good idea to publish a separate document that is the beta training document for the feature. This will provide instruction and get feedback on how best to instruct users in it's use before incorporating the feature into the regular training.