I would probably try to use UTF-8 whenever possible - it just covers more ground and is more flexible than ISO-8859-1 which will choke on e.g. Eastern European characters already (try to write Jiři or something like that in ISO-8859-1 - it'll fail miserably).
So if you really want to attempt to change (which I applaud!), then I'd go UTF-8 and only resort back to ISO-8859-1 if you really can't make UTF-8 work.
MArc
Here from outside english-only-land{1} I can confirm that UTF-8 works fine everywhere and has done so for many, many years. I have trouble remembering since when any MTA crippled emails by stripping of the 8th bit (leading to "inventions" like QP (which were basically fixing the symptom rather than solving the problem)). That happened most certainly during mid-90s, although UTF-8 quickly gained popularity and replaced iso-8859-1. I do not remember when I switched, but I guess it was at least before year 2000.
Speaking of iso-8859-1, it will not be able to cover all possible input from your users. Depending on language, other iso-8859 variants might be needed (for instance for Finnish and Welsh), and even so the 8859 family does not support languages like Chinese. UTF-8 in the other hand should cover everything, so I strongly recommend that to iso-8859-1.
{1} This might bias my experience since any program not fully supporting UTF-8 would be considered crap and tend not to be used here.