The differential backups can be used to control.
1 The number of tapes used in a restore.
2 The amount(and therefore the number of tapes) used in a backup cycle.
3 The number of copies of a file changed during a cycle.

using full-1-2-3-4-5-6-7-8-9-full-.... will mean at most 2 backups will be
needed for a restore (but uses a lot of tape in total).
using full-9-8-7-6-5-4-3-2-1-full-.... will use the least amount of tape in
a cycle but you could need lots of backups to complete a restore.
using something like full-2-1-4-3-6-5-8-7-full-... would be somewhere
between the first two examples.

Usually we do fulls every 2 weeks a level 5 in the middle and the rest
incrementals.

What you really need to do is look at your requirements/bottlenecks and
find a schedule that is best for you ( and your backups/restores).

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