fdisk can be used to resize slices. (actually it is same as creating new slice table)
bsdlabel can be used to resize bsd labels, and growfs can be used than to "fix" already presented file systems on resized label (so you do not need to backup data and recreate fs with newfs).
Beware that growfs is used only for labels with increased size, labels with decreased size are broken already - possibility of losing data is highly present - which can be fixed only with newfs.
If you plan to increase slice over some other one, you must backup (at least) data on slice which is going to be decreased in size. After first slice get increased in size, use "bsdlabel -e" to edit labels.
When editing labels, you should only increase last label size(keeping all offsets same) and use growfs only on that one (last label) - there is no utility that change label offset and at same time keep/change its size.
Of course you can edit labels as you like but I'm 100% sure that decreased labels can be malfunctioning (I never tried this) and increased labels will semi work (some files will not be accessible) until growfs is used on that label(or until newfs recreate fs - loosing any previous data).
So answer is something like no/yes,if you care for your data, otherwise anwser is always yes
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Depends if you are interested in not recreating fs(/not using backup) on increased last label of increased slice. Solution for decreased slice(with labels) should always be backup. Of course if you increase slice and do not edit table of labels on that slice, nothing will got wrong with (that)slice - but you will lost usefull disk space.
I hope that it's not so complicated to understand what I written. (if you use only one label it becomes very simple)
Whatever you do, always make backup.