timzsta
Yep, the A/T is armed during the before take off checks and therefore used to set TOGA thrust (T/O or R-T/O) for take off.
For climb, A/P vertical modes used are VNAV SPEED (note there is no path profile as such in climb, only FMC computed climb speed, hence VNAV SPEED), LVL CHG, or V/S (only normally used to control excessive V/S in busy airspace to avoid TCAS confilcts - 737's climb like lifts when light).
Note the A/T never controls pitch - only IAS in A/P V/S or N1 (climb thrust) in VNAV SPEED or LVL CHG.
On finals the A/T is not normally selected off - usually (operator specific) in either MCP SPEED or ARM.
Must admit I've never heard about the A/T detecting high N1 and selecting TOGA - certainly this could not happen if the A/T is off, it might be the case for A/T in ARM though.
Cheers
Fat Dog