Correct me if I'm wrong but I'm not sure if you/some of the replies understand what is meant by fixing?
By fixing you aren't committing to pay whatever they've suggested as your direct debit for the next year no matter how much you use- you're only agreeing to pay x amount per unit you use. So if they say"we estimate you will use 100kwh if electricity per hour each month and the fix will be 1 pound per kwh (not very likely prices or usages but for the sake of simple maths) so we will set your dd at £100 per month" you are not agreeing to pay £100 per month. If you use more than 100 units you will owe more than £100 per month despite fixing ,if you use less you'll owe less.
Regardless of whether you fix you can still try to use less energy so you can build up some credit and lower the dd amount.
Similarly if you don't fix and are planning on paying less than the direct debit amount your supplier has quoted you to fix, that won't necessarily work because after the price cap goes up in October, and then again in January and April, whatever the standard rate is is going to be very similar, possibly even more than the fixed rate.
Again sorry if everyone understands this and I'm teaching granny to suck eggs but there have been a lot of posts where it's clear posters don't get it
Tldr : fixing your rate is ONLY fixing the price per unit you will be charged, it does NOT mean you will use the exact amount per month the fix offer quoted as your direct debit amount - you could end up owing more or less.
And actually answering your q - yes I did fix towards the end of July- at the time against advice but am.quite glad I did now- seems likely I will overpay more than on standard rate Aug and Sept but then slightly less October onwards and significantly less from Jan if the current predictions come into play.