I second Carpenter's comments - learning to cut in yourself will save time and you'll get a perfect finish when you get the hang of it. Figure out which way your hand is stronger - either working from your body outwards or back towards your body (when doing horizontal cutting in) and plan your work to suit that.
A good synthetic brush won't leave any noticeable brush marks. Purdy brushes are the bees knees and worth the investment, though I did pick up a set of mid-price Harris no-loss synthetic brushes that are excellent. I cut in with either a 4 inch or 3 inch brush and would recommend a novice use a 2.5 inch brush - this might get you out far enough from the edge to finish up to with the roller. Make sure the brush is fully loaded - dip it, brush off the excess on the side of your tub so its not dripping, and you should be fine.
hennypenny - I think you might have gotten confused with what your painter is doing: painter's mate is an arcylic filler that is used for joints between two different surfaces where there may be movement - hence it is flexible. You apply this with a gun, run a finger over it to smooth off, sponge it if necessary, and leave to dry. Then you paint over it - it helps to create a sharp edge between the two surfaces making it easier to get a good cut in line. For me use of painters mate (I know it as caulk) is what separates an amateur job from a professional looking job.