If you are considering using an unconventional therapy complementary to conventional therapy then the question of how the unconventional therapy might interact with your conventional therapy arises.
In many cases there is no reason to suspect that the unconventional therapy would interact with conventional therapies at all. You wouldn't expect meditation to have any specific interaction with chemotherapy, for instance.
In other cases there may be reason to think that a non-toxic substance synergizes with your conventional therapy. For instance there is specific evidence that the non-toxic polysaccharide PSK synergizes with chemotherapy in the treatment of a variety of cancers.
On the other hand some substances may actually interfere with conventional therapy - for instance the vitamin folic acid interferes with the chemotherapy drug methotrexate. Clearly if you are depending on conventional therapy you must avoid things which interfere with its action. I recommend sharing your complementary therapies with your oncologist. If there is a specific objection - "folic acid interferes with methotrexate" you should take it very seriously, but sometimes you will get a generalized objection to anything the doctor doesn't completely know the effects of. I might take that less seriously depending on how effective the conventional therapy is.
The more effective the conventional therapy the more conservative you should be about things where the possible interaction is uncertain. That is to say the more evidence for benefit and non-interference you should require. The cut-off value should be higher.
If you have a cancer which is known to be highly curable with conventional therapy, it makes sense to avoid risking anything which might interfere - even if there is no specific evidence of a problem. But if the conventional therapy is unlikely to cure, then it seems reasonable to be less conservative and to have a lower cut-off threshold. For instance, the treatment I had, IL-2, is known to reduce blood levels of vitamin C to a very low level and in one tiny study, the less the reduction the greater the chance for a response. Because most patients don't respond to IL-2, I would think it quite reasonable to take vitamin C with it. If almost all patients responded to IL-2 then you might want more definitive evidence before taking even the small risk that vitamin C might actually be harmful to IL-2 patients.