Assign your user ID numbers sequentially at all times. You will be able to spot irregularities in user ID numbers more quickly. While nothing would prevent a hacker from assigning himself the next number in your user sequence, you should be able to recognize numbers you have not yet assigned. Ultimately, it doesn't really matter where you choose to start your user ID sequence, as long as you pick some starting point and stick to it. In addition to assigning a user ID number, you will assign a group ID number.
Many system administrators assign users to their own primary group.
Further, a user's group number and his user ID number can be the same. That's certainly convenient and if you have no other particular use for group IDs, then there's no reason not to do this. This information is readily accessible to anyone who chooses to look at it, so it's probably best to err on the side of caution. Don't pass out too much information about your users, especially when you don't know who's looking at it. Most system administrators choose to keep their user directories confined to one place or one file system.