Manually assign DNS server addresses or use ISP assignment?

[#1]

A while back (at least a couple of years ago) the advice round here for fibre connections was to assign DNS server addresses manually in the router settings, because otherwise you'd sometimes get a lookup failure and a confusing BT error message. (As you can see I'm a bit shaky on the details after so long.)
Is that still best practice, or is it now safe (& recommended) to specify ISP-assigned DNS server addresses?
I've just checked my router and I've got 212.159.6.10 set as my primary server and ....6.9 as my secondary. I see that on the website Plusnet now lists the preferred order as ....13.49, ....13.50 and finally ....6.9. If the advice is still to assign DNS server addresses manually, is it worthwhile changing my assignments to ....13.49 and ....13.50, or is it OK to leave my assignments as they are as long as they keep working (which they currently do)?

[#2]

If your current settings are working well there's no need to change them. Load balancing probably makes it immaterial in any case.
As in your case, on my Connection Settings page 212.159.13.49, 13.50 and 6.9 are given for DNS. However the router is using 6.9 and 6.10 and those settings were provided automatically.

[#3]

Thanks. (And sorry for the delay in replying - I was doing other things and forgot to check back here.)