Many of our docs write parameters to ensure the meds get given, not held. When you have a CHFer on the "cocktail" (ACEI, B-blockers, lasix, et. al.) many times they need the meds, even when it seems like their pressure will not support it. If it is a chronic med then the effect will be less dramatic than if it is a new med.
It's nice to have parameters to eliminate some of the gray areas that exist. I've seen parameters for beta-blockers, CCBs, ACEIs, hydralizine, nitrates, even lasix. Many times though there is some nurse discretion involved. For our post-op CABG/Valve patients, the holding limits for beta-blockers was SBP<90 and/or HR <50, more so that we would give the drug in light of lower pressures.
But back to your question, really anything that can change the hemodynamics could have parameters, but most often it comes down to nursing judgment.