I keep meaning to respond to this one as it is right up my ally, but every time I have a few minutes to do so, it's no longer at the top of the list and I forget to look for it.....
It is never a "wrong" time to ask for a bonus. However, there are times (which differ MSC to MSC) when you will be more likely to be granted a bonus and times when the bonus you can get is higher than others.
Here is what I do:
1. If an MSC calls me, I am going to ask for a bonus if they don't offer one. If they do, I might ask for a higher bonus. How much do I request? I have a certain hourly dollar amount for which I will work/drive. I'll do some quick math to determine what that dollar amount is for the shop in question and add a few more bucks. I always make sure that the caller knows what travel is required for me to be able to do the shop - including tolls, ferry rides, etc.
2. If an MSC sends me an personal email or text, see #1.
3. If an MSC sends me a mass email, I read it and try to determine how "urgent" or "desperate" the scheduler really is. If i believe the scheduler's situation to be dire, see #1. Otherwise, I'll likely not respond, unless I was headed in that direction anyhow. If I am headed in that direction any how, I'll make an offer similar to what I said in #1, without adding transit costs (or maybe adding a nominal $5-$20, depending on my sense of the situation).
4. If I am just looking at a non-bonused job on a job board, I don't bother asking for a bonus.
5. If I am looking at a bonused job on the job board that I know to be hard-to-fill, see #1.
6. If I am looking at a bonused job on the job board that may not be that hard to fill, I might ask for another $5 or $10, but I'll consider myself lucky if I get it.
7. If I am interested in picking up a bunch of shops from the same MSC, I'll often request an extra $5 or $10 per shop for doing the lot of them.
8. If the MSC won't meet my price, I thank them for their time and let them know that if they do get to a point where they can pay me what I ask, I'd like to hear from them.
You are, overall, far more likely to get your requests offered at the end of a month or quarter as that is when MSC's have deadlines approaching and when they are ready to really pay to get those difficult-to-fill jobs done. Bonuses at these times can be extreme. However, there are sometimes locations that an MSC knows to be near-impossible to fill and that the MSC has not been able to get filled for a very long time. (I can think of a few towns in Washington where this is the case. Think: Gas stations and Post offices in towns of under 1,000 folks that are a good three or four hours from metropolitan areas. There is one town in Washington that can only be reach by crossing a piece of Canada, so it requires two international border crossings each way!). If you are looking at this type of location, don't hesitate to make an offer at any time of the year for whatever you think it's worth - and don't cheat yourself.
Hard work builds character and homework is good for your soul.