Hmm, I think I might just do that. Makes getting around later in the game a lot easier.
I especially hate it when a friend calls and it's physically impossible to get there in an hour driving.
Just catch a taxi. If you press A to skip the journey you'll arrive instantly. Then just steal either the taxi you just arrived in or another nearby car and grab your friend. You'll have plenty of time to spare.
As for helicopters, you can always grab one from the Helitours place outside of dates or missions. Again, take a taxi there.