Without coming out and saying there is a 'right' or a 'wrong' way to manage your tasks, I will say that working primarily from a master task list is not a GTD-way. My understanding of David Allen's primary concept with GTD is to overcome the limitations of a master 'to-do' list. If one is constantly working from a master to-do list, then everything on that list is constantly competing for my attention. Even with the Today view in Things, I expect that most followers of GTD still apply due and tag filters to that view to work in a contextual mode.
As to how I find using OF most useful, it is working from a context perspective, and my 'Today' list is only a quick overview of what I need to work on as well as what I want to work on for a particular day. For my purposes, it is not intended to be an comprehensive list of what I must complete today.
My Today list is also situational, so for example I may flag 'weed garden' to appear on my list today as part of my daily review. If I don't get to it today, then I may be fine with that. I may also unflag it during tomorrow's daily review if the weather is bad or if I am traveling. Or I may also assign it a due date that it must be done today if weeding the garden cannot wait until I return from traveling.
In my opinion, the main concept of GTD that must be done without fail is the Review process. I conduct a daily review the first thing each morning (some review multiple times daily) where I thoughtfully plan my daily objectives given the contexts that will be available to me, the firm commitments that I have made to myself and others, and the objectives I have fleshed out in my weekly review. My weekly review has also been informed by my monthly review, which is in turn informed by a quarterly review, etc. It is only during the various review processes that I allow all the projects, tasks, and processes that are specific to that review to capture my attention. This is the process that I find most effective regardless of the tool I use-Things, OmniFocus, pen and paper makes no difference. Having said that, the review tools in OmniFocus make it easier to conduct a review.
I don't know what review process you have in place, but I have found that tweaking the Location Services feature of OmniFocus can be micro-applied to the detriment of actually getting anything done. I would find working with my customers from GPS location frustrating. First, I would not want to depend on being in a specific area to remind me to contact a client. If I would happen to be in an area where the client(s) are in close proximity, I may not have the needed resources (product mock-up, brochure, job quote, etc.) with me to call on the customer. Or I may only have an hour available to me, but multiple clients pop up in the area, with tasks that will far exceed the available time. Or worse still, I have the resources, I have the time available, but the client is not available to see me. Applying a location context to an errand that I can accomplish in the same geographic area makes sense to leverage my time, but using location context as a primary planning tool for what must be done is not effective for me.
As a quick thought on the other examples, OmniFocus has some advantages when setting repeating tasks. A task can be set to repeat after an interval (like Things) or can be set to start again after completion. OmniFocus also allows repeating tasks to work on the iPhone, which is not currently possible with Things. I see in another thread that you have asked about repeating tasks with an end date. While I can agree that this would be a nice feature, it is during my weekly review where I would see that the task is no longer needed. I would then delete it if it would never be used again, or schedule it for a date in the future if I will need it again.
As an example, I need to write a check monthly for my children's' school lunches. During my monthly review for July, this monthly repeating task popped up. As they are on summer break, I changed the next start date to mid-August when they return to school. In mid-August I will write a check for the partial month and then change the next occurrence to start the first of September. Easier to do than it is to describe.
With your example of a repeating task with the phone bill, I don't see anything in that task that is actionable? Perhaps I am overlooking something, but I have found that if I cannot begin the task description with a verb, then it is not a task that is clearly defined and actionable. What do you want to act on with this task? If you are making the payment, define the task as 'Pay phone company $100'. If the payment is scheduled to be auto-deducted, define the task as 'Confirm $100 payment to phone company' and then decide what the next action should be if the payment does not happen.
Hope this helps.