What time do you use clearblue digital OPK?

I always thought OPKs were supposed to be used in the afternoon but reading these instructions, it says they can be taken at any time during the day and to have at least a 4hr hold on your urine. It says most women find it easiest to use first morning urine.

So they work just as well if you use first morning urine? I was always told that's the worst thing for OPKs.