It breally depends on the yeast you use as each has its own tolerces towards temps and abv. That being said I choose to ferment my fruit and white wines at cooler temps in the low 60's which helps retain the wines fruity esters and not extract as much tannins.
When you say "dont let it get mid 70's or higher". Does this only refer to fresh fruit wine (like actual strawberries, blackberries, pears, etc..)? Or, does this include wine from concentrate (such as welches)?
at higher temps the yeast may work faster and thats not always a good thing. They may eat nutrients quickly and then stress and then put out H2S or other foul compounds in higher rates like Tom mentioned. Stable and cool is best. In warm months I ferment in a cellar.
I had a peach wine that did get warm recently, it had a rather nasty aftertaste but it tamed considerably after a couple rackings and some time in the bottle. I can still detect it but everybody else likes the wine. In my head I know its not what I went for, so the wine is being given away.