Is this really critical? I know when doing whips and slacks that you need a tension free string. I’m still working on my project (KUDOS to those who still remember when I started it!) for the Yoyo String Trick Ladder and I was working on Iron Whip, and it was nearly impossible to do the whip of it.
On other tricks, does the string affect yoyo performance?
The string GREATLY affects Yoyo performance. Bad string will make slacks and binds harder. Also old string doesn’t slide in between the gap the Yoyo as well
It really depends on your style. If you do a lot of slacks and whips then yea, you’ll want a high quality string that you replace often.
I personally don’t use a lot of slacks or whips so it’s not as big of a deal for me. My string of choice is still YYSL Ammo just because it’s really soft and on the thicker side but it wouldn’t be a big deal if I had to just use YYE poly.
I don’t really think it matters that much at your level as well.
I notice a detract in performance in a string in the first 20 minutes. I replace after half an hour of play. If someone wants me to try their throw and the string is worn, I will decline. Disgusting!
You wouldn’t want to try out their throw? I only live by one person who yoyos and he has a yoyo I have never had (Shutter) so I love trading yoyos with him, so I feel really lucky to be able to trade 
I’ve found that a yoyo the condition of the string affects the play at all levels. Having problems w/tricks you previously had no problems with, change the string.