Totally agree.
YOU think you are gods gift in the role, and you've heard what could also be encouragement as they might perceive a lack of confidence/something.
You also need to be realistic as to what rise you get. If you came in on 18 and asked for 25 after 3 months and I was your boss..... Unless the sun literally shone out of your arse I'd laugh my face off/ tell you to be slightly more realistic and maybe offer 20k....
Not all managers will be like this
I wouldnt necessarily leave it till last minute either, but it depends how important it is to you, if this is your first professional role then it may be the cost of getting your foot in the door
If your review is at 6 months, then at 5 months in make some time to be proactive and speak to him about how you are doing, get him to confirm that you are doing everything needed and more, then bring in to the conversation that you are enjoying it although you perhaps feel you might have been undervalued at the start before they had a chance to see your capabilities
refer back to them casting you into a junior/trainee position and therefore the salary you were offered is 18k
then set yourself aside from that, you have managed everything they've asked in your stride and are managing your own workload and have clearly established yourself to be more capable than a junior
keep it as civil/light hearted as possible, you are asking for something that your manager wont need to give you and your really just enquiring if he is willing to do a little more to keep you happy
by bringing it up in advance of completing your trial, you give him opportunity to get it cleared with HR before hand, who will need budget approval from the beancounters... if you wait till they present you with a contract, at that point its already agreed and they will have to go back with cap in hand, unlikely that it will work outside of a review cycle