If you think that other people would need to edit your vi in LV 5.l, LV 5.1 is probably the way to go. However, even if you go with LV 6, you can always use the Save for previous option that allows you to save in 5.0 format. One of the biggest considerations you will need to take into account is whether you need any of the features that LV 6 provides. You can find a list of things new to LV 6 in the following document:
http://digital.ni.com/manuals.nsf/webAdvsearch/F1272601AF0157C98625693C00501159?OpenDocument
Hope this helps.
Matt Kisler
Applications Engineer
National Instruments
http://www.ni.com/ask