For most people I'd recommend them sticking with the latest stable version (currently 1.4.6).
That version has been around for quite a while and its functionality is well known and documented as a lot of people use it. This means that most people who use it on a regular basis know how it is supposed to work and what weird little "ticks" it sometimes does which aren't wanted but are expected.
This means that if you have a problem with the stable version it's much more likely that someone already knows about it and can advise accordingly. In these cases a solution is usually quick to find and you can move on with your project.
On the other hand, developmental versions are used by a much smaller subset of the Scribus user base (judging by the SourceForge weekly downloads data) and as such, when a problem occurs, there are fewer people that can help which means that a solution can take longer to find. Also, developmental versions are, by their nature, unstable which means that they might contain features that haven't been tested fully or they might do things in a different way.
SIDEBAR: If someone is a Scribus beginner and they come across a problem with a developmental version, how do they know if the problem is a bug or whether it's a new feature or they're just not using the software correctly? They have no baseline for comparison and people who haven't used the latest version won't know about the new features. I've seen plenty of "bugs" reported that are just the result of people not knowing what they are doing.
Not only that, but for some developmental versions you can have a lot of different "sub-versions" all masquerading under the same version number, all with slightly different functionality as bugs get fixed and the changes get implemented into the latest one. This means that one person's 1.5.x which was compiled last week probably won't be the same as another person's 1.5.x which was compiled today.
In cases like this it's very difficult for anyone to bug-fix as it can be very difficult to find/compile the exact version the problem is occurring in in order to see what's going wrong. Because of this someone will probably recommend that you install a copy of the latest version you can find and see if that makes a difference.
So what does all this mean? In my opinion there are three reasonable options:
1. Stick with the stable version. Most people use it and you can get support quickly. It's not perfect and it doesn't have all of the latest bells and whistles but if you're working to a deadline and need answers quickly then that's the best option.
2. Use the stable version until you can go no further, then create a copy of your document and take that over to the latest version to add the "extras" while you still have a stable version of the document you can go back to if you get into trouble. (I.e. You can't take a 1.5.2 document back into 1.4.6; Scribus documents aren't backwards compatible.)
3. Take the time to find out how you can get - and properly keep up-to-date - the "bleeding edge" version (most likely by compiling it yourself). This will contain any bug-fixes that have been made lately and should have fewer problems than any previous developmental versions. However, you need to use that with the knowledge that something could - but might not - go horribly wrong.
To expand a little on what mnawij said above, a lot of people use the latest developmental version without much trouble. No software is bug-free so it's your own choice which path to follow. If you have doubts or worries then install both versions - stable and developmental - and use the stable version for the important stuff while experimenting with the developmental version.
You might find that you prefer the stable version, and that's fine. On the other hand you might find that the extra features of the developmental version outweigh the (potential) problems and prefer that version instead, and that's fine too.
There is no right or wrong answer. Use what you're most comfortable using while being aware of the potential risks either way. With the stable version you're risking not having all of the latest features which could be helping you do things better but, on the other hand, with the developmental version you're risking using software that could bite you at the worst moment.
P.S. On Macs, one easy way of having both versions is to install the stable version in the system Applications folder while installing the developmental version in the Applications folder in your user folder.