I don't think you can really assign a ratio using poundages for abs and lower back (like crunches with a 25 lb plate and back hyper extensions with a 25 lb plate) because its sort of a tough thing to measure, as there are not many ways to use weights equally on abs and lower back.
I think the key is just to do an all-around core workout. While squats and deadlifts do require good overall core strength, the core has more of a general stabilization role. T
his is sort of like if you start benching workouts after a long layoff, your lats may get sore as they're working to handle their scapular stabilization function. While I never really lose conditioning in my abs, when I first start a really heavy squat cycle, the first few days I'll notice some soreness in my abs as they're getting used to helping support much heavier weights on my shoulders again.
But your core does need direct work to really see benefits. I think that this is mainly to counter most people's poor posture in the rest of their lifts.
The majority of people allow their pelvis to have too much anterior rotation. This is primarily caused by tight hip flexors and weak abdominal's that can't pull up on the front of the pelvis enough.
The hip flexors are one of those muscles that tightens with lack of conditioning, pulling on the back crests of your pelvis and lower spine, causing your hips to tilt forward. Your abdominals are the kind that loosen with lack of conditioning, allowing this hip-tilting to happen.
This hip flexor tightness/abdominal weakness also causes your gluteals to become inhibited, and the hamstrings to become overstressed and tight as a result, contributing to lower back rounding at the bottom of a squat and deadlift.
So there are some key points that I like to keep in mind when developing my core workouts and for just core health in general:
1.) Generally, you should try to work everything in equal amounts. This is the closest thing to a bench/row ratio idea. Its sort of like the old guidelines of "for every set of bench work, you should do a set of rowing work as well". Your core muscles are designed to work together in any complex lift or athletic movement, so IMO you should also work them out together.
My core workouts include equal parts abs, back, obliques, gluteal and rotational work, usually done in a circuit type of activity. I use a nice compact circuit that takes about 15-20 minutes. It's sort of like medicine for my core. Anytime my lower back is aching, a few doses of my core medicine and it feels all better.
2.) The goal is to achieve a balanced, neutral pelvic position. In many cases that means correcting any lagging areas, and for most people, that means increased rectus abdominus and tranversus abdominus work. So if this is your problem, I would do an overall circuit, but add some more abdominal work until you're ab strength catches up to your other core muscles.
A good way to measure this is though your pelvic posture. Check out the Neanderthal No More series http://www.T-Nation.com/findArticle.do?article=315nean2. Part 2 gives a postural analysis that should help you diagnose your pelvic posture.
3.) Remember when doing any kind of leg lifts (hanging leg raises, knee ups, etc.) to make sure that your pelvis is posteriorly rotated by concentrating on contracting abs. This is essentially what the often-repeated mental cue of "abdominal hollowing" is trying to get you to do.
Although that cue is a bit misleading (it causes people to think correspondingly "round out my lower back", which is generally bad during a lift), its purpose is to combat that anterior pelvic tilt. This insures any leg-lift movement is not being dominated by your hip flexors and that your abs are activated and resisting the hip flexor's anterior rotation on the pelvis.
4.) Concentrate on proper hip positioning during all other lifting. For example, don't allow too much hyper extending or rounding of your lower back during squats or deadlifts. Also, when doing overhead (seated or standing) movements, keep your hips and back balanced, concentrating on keeping your entire core tight.
Don't allow that huge lower back arch lots of people get when tey're doing military. These will help you prevent core imbalances and injuries.