Stay-Loc's at deck level or at top?

[Brion Toss]

Indeed there tends to be more corrosion at deck level, but note that mechanical terminals at both ends mean you can, if you remember to, invert the wire at its half-corrosion-life, thus prolonging its safe working life. People rarely actually do this. Sta-Loks or similar at both ends are also cheaper in the long run, as you don't need to buy any terminals when it is time to rerig.
Mechanicals are handy at the bottom end if you need to turn up the bottom ends in place, post-stepping.
Fair leads,
Brion Toss