Yes, it will matter.
Since you're talking about end-game content, by being a Guardian you're supposed to be a tank, and that means taunting your enemies, surviving, and helping your group in their damage dealing.
Guardian slash accomplishes two of those three tasks, being a huge taunt and a stacking armor reduction that changes into bonus damage when it's full stacked.
If you're struggling with talent points placement I suggest you to search for talent builds around the web(simply looking for "swtor talent builds" can bring a lot of useful results).
By the way, it seems that the Vigilance spec is better for tanking purposes (I cannot say that by experience, since I've never palyed a Knight). If instead you're not trying to be a tank, I suggest you to try the Focus spec.
EDIT: since you're a solo player, as you said in the comments, then no, it won't make that much difference, except for the fact that you'll be killing thins at a slower pace(and you'll be missing the armor reduction part that might come handy with the tougher opponents)
Playing alone you won't need the threat component of the spell, since your opponent would already be focused on you.
Keep in mind anyway that, should you be grouping for a flashpoint or an operation, you'd be doing your group a disservice going in without the proper points distribution.