A lot of it depends on the viewfinder, as well as the particular scene you're trying to frame.
Smaller, crop-DSLR viewfinders, you might want more focus light. Huge FF pentaprisms, maybe not so much.
And in my case, Sony-A55 with electronic viewfinder, first generation, that 'helpfully' goes black with manual settings and a manually synced strobe (camera doesn't know strobe is there, and is trying to give you a preview of how lit it thinks the shot will be), I need a LOT of focus light or my view is sometimes nonexistant. (All later SLTs from Sony have the ability to disable this 'feature'.)
Current rig: Sony SLT-alpha55 in Ikelite housing, Sigma 105mm f2.8 DC Macro w/ Ike 5505.58 flat port or Sigma 8-16mm f/4.5-5.6 DC HSM behind UWCamStuff custom 5" mini-dome. Dual INON z240 Type IVs triggered with DS51 for TTL mimicry, or DS51 alone with home-made ringflash assy for macro.
Topside, unhoused: Sony SLT-alpha99, Sigma 150-500mm + 1.4TC, Sigma 15mm diagonal fish, Sony 24-70mm f2.8 CZ, Tamron 180mm f2.8 Macro...all the gear and nary a clue...