I find it all the more difficult, if it's a film I saw back in my childhood.
Rumplesink wrote: This I find more interesting. Do I rate films I saw from my childhood by the standards I had at the time? Or do I rate them from the perspective of my current standards and taste, having matured, experienced more and know much more about films, filmmaking and the world in general? If I rated say Police Academy back when I saw it originally (when I was 13) it would be a tier 10 film for sure. I feel confident however that if I re-watched Police Academy today I'd hate it. Yet I have seen it, and I did enjoy it at the time. An interesting quandary, especially when you consider that something like The Blues Brothers - which I saw at around the same time as Police Academy is still a tier 10 film for me.
The conclusion I came to (as much as it matters, which again - it totally doesn't) is that I'll rate it through todays eyes after the fact. If one assumes that the purpose of rating films is to let others know what you're opinion of the film is rather than used to be then this seems like the most reliable way, given the less than ideal circumstances. I'm not going to re-watch Police Academy in order to more accurately rate the film today. Instead I'll rate it according to what I remember about it (I usually remember films very well, even many years after having seen them), coupled with how it lines up with my current tastes. Maybe not perfect, but it does the job imo.
However, this also brings up another interesting aspect of film watching; expanding your cinematic horizons. Some films require more of the viewer than just the time it takes to watch it, and may reward more than one viewing to appreciate all it has to offer. If you had asked me to rate Andrei Tarkovsky's Stalker immediately after I'd first watched it it would have received a pretty poor rating from me. It's now in my top 10 films of all time.
Which is a roundabout way of saying; things change - and your rating's should change accordingly. They're not set in stone.
Good luck in your cinematic journey.