Crooked sits and downs will not matter during the CGC, but they can cost you alot of points when trialing for your CD.
I don't really use a different word for straight sits, but my body language does indicate to my dog when I expect them. For example, when we are out for a relaxed walk, my body language is relaxed. When we are doing formal heeling, I stand upright, hold my left hand against my side and give the cue "get ready". All of these cues indicate to my dog that we are "working", not just out for a neighborhood sniff.
You can use whichever language is comfortable for you in the ring. (I use a mixture of English and German.) Just be consistent and don't utilize phrases that can be misconstrued as double commands.
Are you familiar with training a tuck sit? This can be a good foundation exercise for young dogs.
PAM Guardyan's Gavin, VCD2, CDX, RE, AX, AXJ, AXP, AJP, TD
PACH Guardyan's Helki, CDX, PCDX, RA, MXP3, MXPB, MJP3, MJPB, CGCA
Guardyan's Kamikaze BN, CDX, RE, OAP, OJP
Guardyan's Kricket BN, CD, RN, NAP, OJP