Aggressive option is in fact not recommended, but you could always try it and see if it makes your system faster although I don't think so. Cleaning the registry in aggressive mode will not make your system faster IMO, because the difference between default cleaning mode and aggressive isn't such that the entries cleaned with the aggressive option would make your PC run faster.
Warning: you should let the tool always make a backup before running Registry Cleaner especially when run in aggressive mode. If something goes wrong, you can always restore the backup then.
