Yes. Standard Finish gives you a 1,000 potency attack (with only a 50% dropoff for AOE—not a staggered dropoff like old Holy—so each mob after the first is hit for 500 potency) for 2 successful steps. So you should always use it every 30 seconds for bigboi damage each time. Overwriting the buff isn’t that huge of a deal.
Procs don’t break your combo, so you can use them as soon as you get them. So, for example, if you use Cascade and Reverse Cascade procs, you can hit Reverse Cascade immediately, and Fountain will still stay lit up for you to hit afterwards. Your procs give you chances to get Fourfold Feathers, so you want to make sure you always use them whenever possible.
Giving 10% to a DPS (because the buffs do stack) would be better than 5% to a tank, as DPS should naturally deal more damage thereby making more use out of the buff (and DNC AOE is monstrous—so imagine +10% flat damage in dungeons on top of AOE burst). But it would depend on if the DNC is also doing decent damage to know if this is worth it. I usually Closed Position My fellow DNC in 4-mans. If they don’t Closed Position me, then I may Closed Position the tank.(I have had some that just don’t use Closed Position at all, or give it to the healer of all people.)
Any job will benefit from it usually. Typically, you want your dance partner to be the highest damage dealer, as the crit/d.hit bonus will be more effective on them than someone doing less damage.
Back in 4.0, this buff would have been the best friend of BRD—but they removed BRD’s reliance on critical hit this expansion. MNK may still have RNG based around crits for their chakras, so it could benefit them with building up their chakras.
I placed my gauges near my character, so that I could keep an eye on her feet for AOE. Most of my HUD (crossbar, target bar, focus target, enhancement/enfeeblements, HP/MP) are at the bottom of my screen grouped together. The best advice I can give is to run content and attempt to readjust your gauges around where you find your eyes loitering the most. I’ve moved mine around several times attempting to find the best place for me to put it while also keeping track of mechanics.