There are 2 reasons to fix.
1. Because you want the certainty or
2. Because you think you will save money.
If you want the certainty then go ahead. What you see is what you get.
If you think you will save money, in my opinion don't fix. This is not because I know anything about future rate movements it is precisely because I don't. The banks will not let you fix at a rate that they think will save you money, unless you think you know more than the bank don't fix to save.
It may well be possible to exploit anomolies in the yield curve which distort bank costs over fixed time frames and so save by fixing, but unless you think you understand those anomalies better than the banks and have a better insight into how they will unwind, stay away.
If on the other hand you were just thinking of having a punt on saving by fixing, stay even further away.