Pin Lock Kegs & Blow Off Valves

I scored some inexpensive pin lock 5 gal. kegs and am wondering on the importance of a blow off valve on the lid. Also if I convert two as secondaries do I need different lids? I mostly use extract and steep grains right now.

Sorry if this is a repeat question.

 

Not that important for the lids, more convenience than anything, you just need to manually depress the poppit in the gas post to relieve pressure. You don't need different lids, easy solution to avoid carbing the beer before you want to would be to remove the poppit from the gas post, sanitize a piece of plastic wrap, and stick it around the post with an elastic. You could also remove the post and stick and airlock in the hole. There are DIY's on this sight showing how some changed their kegs into fermenters.