One of the guys I train with in Aikido is a Sandan in Iaido and Shodan in Kendo. He is a 4th Kyu in Aiki - although his weapons works is a bit better than you might expect at that grade.
Anyway - he has run some Iaido sessions with us, and I got a lovely new Iaito at christmas to practise with. The way he teach us to do, what we might call a Shomen blow, in Iaido is interchangable with how we do it in Aiki.
Obviously Iaido add a lot of other sword movements, drawing the blade, the circular cuts, the blood flicking and the like, but the basic 'overhead' cut didn't seem to me any different.
As a side note - there does tend to be a difference in Kendo, when he demonstrates with a Shinai there is more aggression and more ability to shift and feint. Presumably as it is a sparring art not a kata based art form.
In answer to the original question I would say take any opportunity to learn something your interested in, there will be no major detrimental effects and it is possible with study the two or three styles may complement each other.