I suspect the 'piton every 3 feet' rule is a worst case scenario. While true that modern day climbers avoid the use of these 'crude' devices, DnD climbers may not be as eco-freindly nor will they hae all the cool gear that makes them less nessecary.
They also tend to not wear encumbering armor and pointy weapons
If I was running a group up a high
sheer face, I would propably stick to the rule of a piton every 3 feet. As the slope's pitch decreases and quality of natural hand-holds/niches increase, the reliance on pitons would slide down the scale rapidly.
Running lead it is common to place 'protection' every 10 feet or so. {protection is a point placed to tie the rope to the rock face, either a piton or more eco-freindly modern gear}
One of the main concerns in placing protection is when you fall, you fall past the point of your last protection... so placement depends more on how confident you are that you wont fall than in any set rule of placement.
IMHO, unless you have a group of climbing gamers, it would be best to gloss over the details of the climb and assume the character with the Climb skill knows how/when to place this stuff. If falling off a lead, you could use the amount they miss the climb check by to determine how far ahead of thier protection they had gone.
In essense dealing subdual damage for falling twice the distance they failed by.
You could also use the distance of the fall as a STR check to see if the protection {or belay man} can stop the fall. Failure results in another drop of 1D20 feet to the next protection point..and repeat. The falling climber only takes damage at the end of the failures..or when they find the ground.
Really the only time you should see pitons in action inside a DnD game will be jamming doorways

A full blown 'everyone is climbing' 200+ sheer face climbs just don't feature that often.
Usually the Rogue prances up the short cliff-face and top-ropes for the less skilled party members. {okay, that might be an exageration

}
Caveat, Rappelling/Climbing for 20 years and I top out at a 5.8 route and have only done one short lead climb to date. Folks with more experience at this are definately out here.