It depends from civ asking for help, and their plans (?) :
In my current game as Netherlands, Shaka requested joining to war with Shoshone, I agreed thinking "This will make Zulu not send their forces on ME!" (also, Shoshone had nice city and I had economical problems). We destroyed Shoshone in giant battle, Shaka razes two cities
I capture Moson Kahni, 10 turns later Shaka attacks... me
To sum up, Zulu ambitions brought nothing but despair and bloodbath to the grasslands of the East Continent.
I have similar experiences with Rome, but also cases of awesome companions - especially... Catherine.
While talking about warmonger penalty, I have bad news - as I have taken Shoshone Capital aaand unfortunately "their last city" I have now penalty with Portugal (impossible to talk with Maria), Arabia (possible to talk, but not as friendly as normally) and for short period of time I had penalty with... Zulu! But it disappeared after... my war with them :rollereye: Also there is Persia who surprisingly doesn't care about my "warmongering".
I love one thing (I don't know when it was implemented, certainly not in the Civ5 base game): if you warmonger in the early game and later meet new civs, they don't care about your ancient "unseen" wars.