Battery PSU with voltage meter.

I am in process of building external battery based PSU for my Chmoy headphone amp. I want to use it with two 9V batteries or 16 AA batteries. The case will accomodate both. The 16 AA batteries will be used with 2 RadioSchack battery holders ( 8 batteries per holder, which would give me 24 volts to split). The reason for this is practical... I will be able to use AA rechargeable batteries ( RadioSchack part number 2700387). I want also include analogue volt meter. This is for both... aesthetics and practical info. The use of volt meter is what I need advice for... can I use a volt meter constantly connected to batteries or should I use switch to be able to turn it on only when I need it? I am not sure about battery drainage if meter would be constantly connected. The meter is 15 volt one.