No, it is not. NOCACHE indicates that values of the sequence are not preallocated. Minimum CACHE value is 2 (therefore, you can not specify CACHE 0). If you omit both CACHE and NOCACHE values, Oracle caches 20 sequence numbers by default.
Optimum cache value? No idea ... but, thinking about it, it seems that if sequence is often used in the application, there might be some advantages if larger number of sequence values is cached.
Depends on how often the sequences are going to be used. If the default of 20 is used, Oracle will read the next 20 numbers and load them into the CACHE.
One point to remember, if you plan on using the sequence number for any type of reporting, keep in mind that you may have "gaps" in the numbers.
ie, Oracle loaded the next 20 sequences, the database is brought down for some reason ... Those last 20 sequence numbers are now gone...