The below is from my Flight Crew Training Manual (published by the manufacturer). The at-or-below paragraph is of interest. Using the technique below provides altitude alerting, and if I come out of VNAV while maneuvering around a buildup, or for any other reason, I will have altitude capture capability. I am in agreement with Cough in that circumstances will determine which mode I use.
Climb Constraints
Climb constraints may be automatically entered in the route when selecting a
procedure, or manually entered through CDU entry. When the airplane levels off
at an MCP altitude, that altitude is treated as a climb constraint by the FMC.
All hard altitude climb restrictions should be set in the MCP altitude window
including “at or below” constraints. The next altitude may be set when the
restriction has been satisfied, or further clearance has been received. This
procedure provides altitude alerting and assures compliance with altitude
clearance limits.
If VNAV is maintained as the active pitch mode throughout the climb, the highest
altitude restriction may be set in the MCP altitude window. The climb must be
monitored to ensure compliance with all altitude constraints. If at any time the
active pitch mode is changed from VNAV to any other climb mode (e.g. FLCH,
V/S, etc.), the next altitude restriction must be reset in the MCP altitude window.
Note: Use of CLB DIR deletes constraints from the LEGS page.