The standard C41 times make the assumption that there is a pre-wet. Without a pre-wet, the developer activity is different (higher I think), therefore you would perhaps need to adjust your times slightly.

Yes, you do need to be careful to drain as much as possible of the pre-wet in order to not dilute the developer. However, I don't think it's a significant problem - my measurements show that there's maybe 10-15mL of water left in the tank (stuck to spirals and walls and stuff) which against the 1L of developer I use, is only 1.5%. The developer (Fuji) gets reused about 5 times, for a total dilution change of 7%, which is IMHO basically irrelevant and certainly less than the effect of exhaustion for which you already need to apply a time extension.