The PhD students I remember the most are the ones who came up to me and made meaningful comments or suggestions regarding my work. They get extra bonus points if in the middle of the night the next week they offer more meaningful comments or suggestions.
Here's the question though: how many PhD students actually manage to make meaningful comments or suggestions about a professor's work? And how often does the professor follow up and inform the student that those comments are helpful (rather than pretend that the comments are helpful as a matter of politeness)? And if the comments are implemented, does the professor ever notify the student?
I'm saying this as someone who makes a lot of suggestions/comments to other people, but who can never be sure whether or not they find them helpful. Most suggestions seem to be discarded simply because it takes too much time/effort to implement them.
If a professor finds your comments insightful and valuable, he/she would take the initiative to request your opinion or collaborate with you on a project. If not, they won't make an effort and may keep silent about it to be polite or outright request that you not give your opinion. Just keep in mind though... Some professors may regard your comments as being a bit ostentation or arrogant... I know, because I've been there and done that myself :)