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Don't think it matters much whether you allocate up or down, however assigning address blocks for certain purposes might be better. For example all 4 sites could have a /21 address block; each site's block is allocated similarly. If the sites differ so much in size that you don't want to "waste" a /21 on each, then you might have a range for /21s (all under the same higher aggregate) and another block/aggregate for smaller sites.
What you want to do is have an address assignment scheme such that you can pretty easily form logical address tree structures where you can aggregate address blocks.
For example each site has a /20, buildings /22s, floors /25.
