Yardages on sprinkler heads are one of the factors contributing to slow play and in my eyes should be avoided entirely. First it takes time to find them and pace them off to the ball position and then people are led to over-engineer their shot, when they see odd numbers such as 121/127/132. A 150 stake is a rough indicator and good enough for most everyone's game. But the sprinkler head numbers, especially if they come in three's, involuntarily make golfers try to take advantage of the pro-level granularity and they start to think about 121 vs. 127, factor in the wind, but they have to carry the bunker... it makes for some grandiose micro management and then they top the ball into the lake anyway.
The stakes are real time-savers and they should be at 150, 100 and especially 50 meters (not sure about yards). Often people over- or undershoot the green with a partial wedge from inside 100, because they thought 45, but it was more like 55 to the green. It's a great time-saver, if everyone hits it relatively close to the flag. So the 50 meters sign is very important (although it seems I am fairly alone with that opinion).
If the stakes are a maintenance problem (or an eyesore) they can easily be replaced by a stone, bush or small tree. If that is considered to be "not pure enough" then lose the yardages entirely. That would still play faster than having them on sprinkler heads only.
Ulrich