On Porter Cable type collets you do not want the bit bottomed out because the bit will chatter. The collet is cut in a W pattern and the router motor shaft hole is inclined slightly. When you tighten the collet nut you are squeezing the W together holding the bit slightly pulling the bit down. The bottom of the hole can be not perfectly smooth causing the bit to spin slightly of center introducing chatter. Look at the collet you are using and insert the bit at least below the clamping jaws. But do not bottom out the bit. If your bit is too low in relation to the table move your router up in the mounting of the lift. If you are using a regular router base then you may need a bit extender. Just remember the further your bit is above the router can introduce chatter with the bit turning out of round. Most routers are turning 16000 to 20000 rpms. Small errors turn into vibration and make a rough cut.