When is Lightmapping bad for performance?

Looking to up my performance, I'm exploring lightmapping. The manual says basically it's a good thing because dynamic lights on objects will slow things down. Ok, but adding lightmaps also increases the texture usage. I'm a bit worried, esp on iPhone and Android, that the resulting lightmaps will blow my memory out.

What do you recommend?

Use as low resolution for the lightmap as you can get away with. Typically texel size for lightmaps is far larger than you'd use for regular textures, but since the lightmaps are just shadows and general lighting information rather than detail, you can get away with it. The built-in lightmapping in Unity will tell you the texture size and VRAM usage; you can always resize the generated lightmaps later (or rebake at a lower resolution).