Is it common practice to give soldiers an award before the 638 is signed?? Is there a reg that explains if this okay or not??

If the approval authority has authorized the award, it may be "awarded" as an impact award prior to the DA 638 being signed and processed, the award is valid as any other.
The sticking point is, when is it counted For promotion points..that will be based on the date the DA 638 is signed and processed...not the date it was handed out.

Sergeant Major, even if they're using an improper method with which to award personnel, wouldn't, if he was awarded by the approving authority, and processed through the right channels, be awarded the appropriate documentation? If a Colonel awards me for a significant action i would hope they know enough about policy for it not to get forgotten about and my being left out.

SFC (Join to see) - you have two issues..
one is ..Is it proper for the approval athority award an ACROM to you on DEC 1 and process the DA 638 on DEC 28.. the answer is yes IAW AR 600-8-22 Section 1-21, Impact awards. ideally, the DA 638 would be processed the next buisness day....but there is no regulatory guidence i know of that stipulates the 638 must be signed and processed in a specific time.

Second: When is that ARCOM valid for promotion points? Answer, after the DA 638 has been signed, processed and uploaded to the promotion system.
The two things are not mutually exclusive

This is true, still some missing details as to the validity of the concern.

WO1 (Join to see) the premise that one arcom may have made the difference between promotion and not. it would depend entirely on how the promotable's points are. if one has 420 points and points are 449 but there are 15 people between 421 and 449. having 30 additional points may not necessarily have found himself on the promotion list. the points may have been adjusted to 451, just above the last person on the list to meet the appropriate amount of slots approved.