question about file locking (flock)...

ok, when is it necessary to lock a file?...

I assume if a file is used only for data, meaning scripts will read data from it but never write to it it does NOT need to be locked. Is this correct?

Now, assuming the above is correct, if a my script is accessing a file to write to, and other processes may need to read from or write to the file, i need to lock it, correct?

This is where im most confused, if my script needs to read from a file, but other processes may be writing to the file, i do not need to lock the file because the writing process will have already locked the file (see above) correct?

or just to be safe should i lock a file whenever its accessed, read or write?