From: Vermont, USA
As a Union, the draft is really mandatory. Keep in mind that during 1861/1862, the cost of Union militia in terms of population is relatively low. In these years, the draft represents a call for volunteers and later a real "draft".
I think a well played Union can have 1 Draft (on Turn 1) in 1861, then 2 Drafts in 1862. You do want the Emancipation Proclamation, but that first draft makes getting the points and victories to get it much easier. Generally you should be entering 1862 with enough PPs to start your second draft and still be close to 1,000 so that your first SV can allow you to declare the EP.
Keep in mind the Confederacy in effect gets a "free" draft during 1862. If you don't use the draft/call for volunteers at all, they will end up keeping things too close manpower-wise. Note also that if you let your PPs get low, the cost of your militia will increase, thus drying up new recruitment as your population sees the war failing and becomes more reluctant to join up.