This is a very common (and good) way of doing it. This is the example given in the Cisco Press CallManager Fundamentals book. I usually configure blocking patterns instead of leaving it out and hoping that nobody comes along and save an overly permissive route pattern.
A big benefit to the device+line CSS method is with extension mobility - when a person logs into a phone, their CoS and line follows them but the route group used will come from the physical phone and thus route correctly for the site. This means if you can visit a remote office and sign into a phone, your calls won't still try to go through the gateways at the home office.
the main reason behind user based CSS or Device based CSS is due to how yur dpeloy the Cluster. If you have a cluster with oly one customer, user then eiterh way is ok however if it is a hosted or a multi- tenant install then the Deviced based is best. The Deviced Based CSS reduces the number of partition and CSS required. If yoo wish to reduce it even further look at "flat partitioning"
We at CDW Berbee do this on every one of our engagement (and there is many :-)) This is a very good way to limit the amount of configuration as with 4 classes of restrictions you decrease the amount of CSS and PTs by the factor of 4. Obviously the more sites you have the greater the benefit. With that being said I do this even on deployments with single site as you never know if a customer decides you add more sites, and this approach has some benefits such as native support for EM, etc.
To answer your other question, the CSS applied to the line has precedence over the device CSS, so in case you have 2 exact route patterns the one matched by line CSS (blocking) will be matched first.
The SRND actually has this approach very well documented as well.
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