The short answer is, it depends on why the hard drive light is on. The long answer:
But today cache algorithms and hard drive performance and caching
changed a lot.
Yes, this is true, but that's not why you waited for the HDD LED to stop (or rather, this isn't the reason that you don't notice the performance degredation as much anymore). Hard drives are big, mechanical devices which are good at one thing - only doing one thing at a time. You can only read/write to a single sector at a time, so attempts to use the hard drive simultaneously usually result in thrashing.
So the question is, do i still am doing good by waiting the hard drive
activity to stop, or could i just do whatever i want and don't worry
Again, it depends on what the HDD is doing. Why? Well, if you're transferring gigabytes of files to/from your hard drive, then you will most certainly experience disk thrashing (and this will also cause the drive head to move back and forth rapidly, decreasing the drive's lifetime). However, if you don't know why the HDD LED is flashing, then it is probably a background process.
Most background processes (search indexers, disk defragmenters, the .NET runtime optimizer, etc...) do not cause thrashing for two reasons. The first is that they run at a lower priority, so the operating system places a lower priority on their I/O requests. The second is that most of these background processes either stop or reduce their activity when your CPU or disk usage reaches a certain threshold, to avoid impacting your perceived performance of the system (or cause thrashing).
I ask this because my latest lenovo notebook don't have the HDD led,
so i don't have a clue when it's working or not.
Don't worry about it then. Most background services in Windows 7 are aware of the issues I outlined above, so you will not experience any slowdowns. Unless you are trying to use the hard disk simultaneously (i.e. transferring a lot of files while trying to load multiple programs at once), you should not experience any thrashing.
Lastly, if you want to know what transfer rate a particular process is using your hard drive, see the question How can I view what percentage of my hard drive bandwidth is currently being used?
One last thing to note, having a solid-state drive totally changes the disk thrashing game. While it cannot be totally avoided (yes, even with a SSD), it will be a lot less noticeable with a solid state drive versus a mechanical hard drive (see What are the pros and cons of a solid-state drive? if you want to learn more).