It's a good idea to avoid trees anyway - falling branches from snow or ice storms can snap a wire antenna very easily.
Let's talk a little about the length - in general, the longer the antenna, the more directive it becomes off its ends (this depends on several factors, such as the ground conductivity and frequency in question). If you have the real estate and are interested in sigs from a particular area, that's one way to go - but if you don't, there are other broadbanded designs - such as the Ewe from DXing.info - that will provide directivity with much less space involved.
There's nothing 'magical' about the length of an antenna unless you're talking about resonance at a particular frequency. And since I'm taking for granted that you plan to listen to more than just one frequency/band, the length doesn't necessarily provide any particular benefit. Especially for VHF/UHF, height would do more 'good' than just length. Now if you could stand that wire straight up (yeah, right!), it'd be a very nice antenna! So running that wire over trees is 'nicer' than running it through the trees, sort of, if that makes sense. The idea is to get at least part of the wire up where the signal is. (See where that's going?)
(Just FYI: Horizontal antennas of some random lengths (long ones) are directional off the ends as already said. That length is in relation to the wave length of the desired bands, not in just feet or meters. Two or more wave lengths is considered a "long wire", anything less than that is a "random wire".)
Considering where you live in relation to the origin of those signals, trying it is about the only way you'll tell for sure if it's going to be any benefit to you. That's a whole lot easier said than done!