[Q] Is Juice Defender necessary using a custom rom?

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Hi,

If I install A revolution or Energy ROM, will juice defender improve battery life further still?

Yes it will. Keeping data / wifi off when screen is off will improve battery life on any phone / rom.

So if I have email, I will no longer receive it instantly (every 15 min is actually not fast enough for me at work)

The whole point of Juice Defender is to cut your Data connection when not in use. Since 15 min is not enough for you, you may as well leave JD disabled. On the other hand, you could go in to the "advanced" or custom settings within JD, and configure the frequency in which it will ping and download email messages,