Well, Dancing blade has never spiked for me, in brawl.
Vertical Spacing: When Marth Dare to Dair.
Dair is the best now. Vertical Spacing is the gimmicky name of Marth's Dair. Really... you learn to implement your 2nd jump according to your Dair (basically creating a RISING DAIR) this makes SURE YOU DON"T FAST FALL TO DOOM...
This method is for quick pace gameplay and even mindgames.
A set up for the type Dair is to dash off (or jump off once) and DI and FF according to where your opponent is. LEARN your Dair Spacing. When you feel that you will hit. SECOND JUMP and Dair.
If you're using your backside you don't need to be above your opponent, but beside them. Either way, Marth's Dair will spike or hit them away from the stage (on contact) at starting at 120% if Dair doesn't spike it should still kill via knockback.
If you are above the oppoenent and initiate the 2nd jump and Dair--occasionally it will foot stool spike which give exact spacing for a spike.
Dair as an edgeguard:
Much like Bair edge guard, face inward and jump off (or remain above the stage) The back side of Dair is easier to Tip with than any other side. Actually I never Dair except when I know I'm using the back side.
Sometimes, I Fmash a character across the stage. Characters with a linear 2nd up and Up B can be quickly killed if you (I) dashed and RAR a SH DAIR. (though you can just normal Dair them, but again, backside is a lot easier and has a bigger hitbox)
A setup that some people will fall for (not typically competiveplayers) is the pivot grab -> down throw -> Dair.
Another thing is the Usmash. Like most Usmashes it slightly slides (even without boost smash; since marth doesn't have one or one that is useful) Typically, if its a big character, Usmash will Stage spike. There was an old thread about it. It wasn't too useful, but Knowing that Usmash can stage spike is useful. (careful if they roll, attack, or get up, since you'll be left vulnerable.)
Beyond that. Learn to space with Dair, and FF SH DAIR (off stage) to get proper vertical spacing.