What's the rule of thumb for power and rpm in a constant speed prop?
The answer to your question is actually really simple: do not exceed the limitations specified in the POH.

Really, that's it!  Note that LIMITATIONS are not the same as RECOMMENDED PROCEDURES.  Recommended procedures are just that: recommendations.  Limitations (exclusively from the "limitations" section of the POH) are the hard and fast rules you must adhere to.

Full RPM in Cruise
As long as your POH does not impose a limitation on takeoff RPM (the Seneca II has such a limitation, for example), then there is nothing wrong with using max RPM from takeoff to shutdown. You won't damage the engine.  If the POH provides a limitation on engine RPM (specifically in Section II of the POH), you must adhere to that limitation.  It's common to see a 5 minute "max RPM" limitation on some turbocharged engines.  Great, takeoff at max RPM then pull the engines back to the top of the green arc until you land.  Easy! 

Manifold Pressure &amp; RPM
Mike Sowsun said:
"You should always follow the manufacturer's procedures but a general rule for direct drive normally aspirated engines is to keep the manifold pressure less than the RPM for cruise power. (eg. 24 inches of MP at 2400 RPM)."

If you decide to apply "rules of thumb" to all phases of flight you will be voluntarily handicapping your airplane's full potential.

Some definitions:
Oversquare - manifold pressure > RPM (23" @ 2100 RPM)
Undersquare - manifold pressure &lt; RPM (23" @ 2500 RPM)
Square - manifold pressure = RPM (23" @ 2300 RPM)

Do not embrace the idea that manifold pressure should never, ever exceed the value of the first two digits of the engine RPM (sounds pretty arbitrary when I put it that way, right?).  It actually may be beneficial to run an engine "oversquare" in some cases!  This is another example of why knowing your aircraft's "Section II" limitations is important.  If 23" @ 2100 RPM falls within the limitations, go for it!!  You won't hurt the engine.  Same with 25" @ 2300 RPM or 26.756431" @ 2196.454634 RPM.  I think you get my drift.  As long as your manifold/RPM combination falls within the limitations set out in Section II of the POH, you're golden!!

Now, climbing out on a hot day with a garden variety Lycoming or Continental (like in 170)?  29" @ 2100 RPM probably isn't wise.  30" @ 2700 RPM?  Yeah, that's better!  25" @ 2500 RPM isn't bad but it's no better or worse that 26" @ 2700 RPM or 30" @ 2650 RPM.  Remember, there's nothing special about "square" manifold/RPM values.  I'd routinely run the IO-520 in my Bonanza at max manifold pressure and max RPM from takeoff to cruise and simply leaning to an EGT target as I climbed (and watching CHTs, of course).  Only when leveling off in cruise did I pull it back to an economical setting (sometimes oversquare, sometimes undersquare; always lean of peak).  The engine ran beautifully with good CHTs and incurred no unusual wear.  In fact, the engine's health improved.  Exhaust and plugs became noticeably cleaner and compressions improved.

In cruise, set the power to any combination of manifold/RPM you desire provided that setting falls within the limitations in your POH.  You won't hurt the engine with a valid manifold/RPM setting.  It's possible to put undue stress on a motor with a bad mixture setting, though.  POHs are notorious for giving poor guidance with regard to mixture settings.  Mixture settings require a little more attention above ~65% power but that's another topic entirely.

I recommend that everyone with engine questions read John Deakin's "Pelican Perch" articles.  He's THE source for fact based, accurate engine operation information. He happens to have touched on this "oversquare" myth here.  See Myth #2.

Edit: I credited John Deakin with my linked article.  I was mistaken.  The author of the linked article is Linda Pendelton.  She links to a Deakin article, though.

Mike Busch also addresses the oversquare myth here.