I like 'em all too, but tend not to buy below 30/35 and prefer 45 or better. Always plenty of notes out there in that range regardless of budget!
I think it's a good idea to get the best note one can but in terms of a minimum grade (VF, XF or whatever) it really depends on what one's interest is/objectives are doesn't it? I'd love to have the luxury of limiting my purchases to higher grades - and obviously have some - but I focus on a few specific geographies in depth. Even on small size sometimes/often the best or even only note may well be a ratty VG or so. Generally waiting for a nicer example worked out ok, but early on I passed on a couple of St Louis metro smalls with problems and those are still missing in my collection. Now if something fills a "hole" (as I have defined it) I generally do my best to get it and, if necessary, hope to upgrade it later. Waiting for a better note would result more in an accumulation of nice quality but fairly common notes rather than a meaningful collection. If I did a type set, trade names or BB $5 layouts my criteria of minimum grade would probably be different (altho the only circus poster BB $5 on MO that I'm aware of is a fine or so). While I'm not ecstatic about my charter 28 note I've only had 2 bites at that charter in roughly 30 years of collecting and was basically out of the market when a better one came along. C'est la Vie.
Always interested in St Louis MO & IL metro and Evansville IN paper money, scrip, financial docs, banking ephemera, etc. Also collect Latvian 1915 - 1940 city and national paper money by block letter/variety