We are blessed with Reiki in that there are no contraindications, no way to imbalance our clients, no diagnosis necessary or recommended. When we do Reiki for others our job is to be open, loving, and hold space for the Reiki to flow and for the client to receive as much as they want. In this respect Reiki is a very safe, balancing technique that we never have to worry about outcomes or whether or not we are doing the right thing.
This changes if we begin to mix Reiki with others healing tools. I often get asked about using crystals, aromatherapy, healing sounds or bowls with Reiki and I always caution students to only mix these other modalities and tools with Reiki if they are experienced in using and understanding the modality they are mixing. The reason for this is that other modalities may or may not work to balance depending on what you are doing.
Most tools require some knowledge. Take for example crystals. Crystals each have different properties and they have to be used mindfully and knowledgeably in order to bring about more balance. You need to know firstly the properties, and secondly the issues at hand. Some crystals such as rose quartz are gentle and nurturing, others such as Pietersite can provoke release of negative emotions in a rather powerful manner which when mixed with Reiki could create a bigger detox than perhaps expected - in general using any crystal with a Reiki healing session will amplify both the crystal healing properties, and the release from your client. Many crystals are toxic to hold, and should not be made into elixirs. We often look at crystals as rather mild mannered and may be tempted to throw some under the table before a Reiki session for good measure, but this is not necessarily a good idea. Do your research, make sure you yourself have a good enough education and connection and respect for crystals before using them with Reiki for other people. By all means experiment on yourself, but when treating a client, always be mindful that when you start to mix and match, the balance and correct diagnosis becomes your responsibility.