Trendlines and channels, as well as support and resistance levels, are more an art than a science. You draw them according to your own visual analysis to help define their approximate locations and positions.
Nothing is so rigid that it has to touch x number of points and traders will draw their lines in a number of different ways according to their own preference. e.g. some traders will join extreme ends of wicks, some will prefer passing through the biggest number of wicks, leaving some under and some over the line, and others will join the candle bodies rather than wicks - it is all up to you as the trader to decide what works for you.
But, think about it - the real forex market is absolutely huge in volume and millions of transactions are being made all the time for any number of reasons. So the market is hardly likely to be checking all the time whether it can trade because the rate might be outside Lexzykool’s channel lines!!!
So remember your lines are actually really a zone around which price might pause and might reverse, but there is no rule that says the market must simply because you draw a line somewhere.
In your second question are you really asking if you should use a ruler? I guess I have to assume you mean the ruler on your trading platform and not a physical ruler to draw lines on your screen?