I use regular cast most of the time, and keep Smartcast on my Shift modifier.
There are far too many situations where I like to keep my skillshot display up to see the exact trajectory. In particular, using long-range spells like Lux/Ziggs is very difficult without the skillshot display.
I play mostly skillshot-based champions so this comes up quite a bit. Remember, Smartcasting is about personal preference; you're not any worse off for not using it.