What will happen if i do a
ifup wlan0 using cron once every hour?
Will it cause problems if the wifi is already up?
No. As @ppumkin said, ifup check if the interface is down first, exiting is already up. Second, there are some driver problems with some wifi module. When you turn off the interface, the driver is removed on the fly, and it's linked again when you turn on. Sometimes this second or later insertion doesn't work.
If you have such device, a simple solution is reboot the system.