Assassin question

It doesn't matter.

Scroll for whichever is cheaper.

On an assassin, there are only a few slots (ex: earrings) that you're able to scroll for DEX or LUK.

Read more: Scrolling for Secondary Stats

 

to give you an idea, I created a poor mode shad (at least in the beginning) I needed 60 base dex to equip the dragon khanjar (not counting the event gear), if it were a craven instead of the dkhan, it would be 10 less dex, as I also need str to divide str/dex, if it were just dex I would use a boot with more dex for example, but, with an average of about 100-120m (counting the price of scrolling the helm) you can keep this base easily, unless you think about MoN (which is perhaps more affordable and better?) 50-70 base likely

 

Because people will either choose to play suboptimal or simply make mistakes.

There's some small nuances between scrolling for dex or luk for thieves other than simply the cost of scrolls. For example being able to keep your weapon equipped after maple warrior gets dispelled or tertiary stats like speed and avoid.

Typically you probably want to go for the cheapest option that gives the most primary stats.