Do I need to tell the driver before the bus ride or is it fine to walk to the front of the bus when close to Udom Suk bts?

I would tell them before.

But, I have never taken that bus when a few people did not get off at Suk.

 

With this, on a very high traffic time and day, it actually is faster to get off there then go all the way to Ekkamai. I have waited another 30 minutes in heavy traffic from Udom Suk to Ekkamai.  The skytrain would be faster.

Only works if you only have hand baggage.  If you have anything in the storage you will have to sit it out until Ekkamai.

Most of the bus gets off at Udom Suk so you don't need to say anything