MaxQ:
There are a lot of pros and cons to doing this. When I was a 727 f/o in 2000 sitting Postal Reserve flying 20 hours a month, I started to get a little stir crazy to fly, so I picked up a Lear PIC gig at $400 a day out of Farmingdale (I really prefer the corporate flying lifestyle and this operator was as close as it comes to that in the charter world). Back then I had 16 days off and usually only worked 4 or 5 out of my 14 on anyway, plus I ended up dating one of our Lear dispatchers, so life was fantastic! Almost like the 727 was my part-time job! Didn't really NEED the extra money, just wanted to pay off some bills and have more "play" money for vacations, all of which is probably why I didn't get burned out doing it.
However, when EOI died and I ended up at Pinnacle, I went to the C.P. and asked if I could keep doing it - he had no problem with it at all since reserve guys USUALLY average around 25-35 hours per month here, but with only 10 days off a month to do it in and the extra dating "perk" removed from the scenario, I got burned out... FAST!
Now, I have enough PIC time to where it wouldn't matter if I bid back to CRJ F/O, hold a 15 or 16 day off line, drop down to 70 hours and have 20 days off, and fly Lears again on those days, but I'd only have 30 hours to spare (our contract only allows us to drop 5 hours below guarantee). Some months in the charter world, that's plenty, with others it might be pretty close. But the big questions are:
1. Assuming you want to get to a major, how would they look at that move? Would they think you weren't happy enough flying Part 121?
2. How long can you fly 30 days a month without getting burned out? Everyone needs down time, even when all you care about is strapping on that jet and getting into the big blue! Eventually, it'll catch up to you.
3. Your profile is pretty sparse, so I have to ask: do you even work for a regional yet? Have you made PIC yet at the regional? Do you have that magic 1,000 hours PIC Part 121 turbine? Is your holdup to upgrade flight time or seniority? You might want to consider picking up open time or trip trades to get your flight time Part 121 so you can upgrade and get that 1,000 PIC faster rather than picking up another job.
G200 has a valid point, that's why I never accepted an F/O position at a regional, but the reality is that most people have and it's not going to stop, so figure out what you need to do for income and do it (G200 also has my dream job - hand it over buddy!)

You also stated a valid point about staying out another night... or two! The good news is that most 135 operators don't tolerate a static aircraft unless the person chartering it is willing to pay for it, so that would probably be a fairly infrequent problem. As far as busting a checkride? Please...

You can't obsess over things like that - giving into those kinds of fears will always keep you from doing the things you want to do in life, just make sure you're always prepared for them and go for it!
Long story short, if you're not doing it for cash to survive and/or you're not going to get time off to "relax and reset", I wouldn't do it.