The RRS only require that the starting line be described in the sailing instructions (rule J2.1(6)).  So the required position of the start would be whatever matches the description in the SIs.

As for what is preferred - it depends.  Different configurations are appropriate for different fleets and fleet sizes. Sometimes, there is no "start boat" - starting off a dock or breakwall for example.  In general, the larger / more aggressive the fleet, the more complex the starting line becomes.  Start boat and a buoy work reasonably well for fleets of <30 boats.  A start boat and anchored pin boat work very well up to about 60 boats.  A 3-boat line (center signal, two anchored pin boats) work for very large fleets, but logistics start to become a significant issue - coordinating 4 line sighters can be complex.  Another 3-boat version involves a signal boat motoring upwind above the center of the line, with the starting line between two anchored pin boats.

You also need to factor in what the class is used to and what they want. 505s prefer rabbit starts for their large events.  Other classes prefer to split a large fleet rather than try to start on a very large, complex line.

There's nothing that says you couldn't put the start boat on the port end of the line, but I think that most competitors - including myself - are used to having it on the starboard end.  There may be specific reasons for that, but I'm unaware of them.

In some cases/areas for large classes, there is a signal boat that roams to windward of the start line so competitors can see and hear signals better.

Signal boat and its anchor line at port end of line (downwind end for starboard tackers) is just asking for damage in a fleet of anything larger than Optis.

Unless of course there is a substantial cross current running from the port end.  In that case it seems wise to put the signal boat at the port end, especially if it is a borrowed boat.

It's not uncommon to have an anchored pin boat - as the pin - on the port end.  Two sets of eyes sighting the line and the ability to adjust the angle of the line with the boats' anchor line make this a highly desirable configuration.  It's also not uncommon to fly recall signals from the pin boat (although coordination with the signal boat can be a problem).  A proper kellet weight to keep the anchor line straight down is a must.

As long as the SIs describe the line as between the flag on the boat and the appropriately described pin, I don't think it matters if the course diagram shows the signal boat at the starboard end.

Matt, I would like to see PFDs on those line callers.
A free-floating line-caller boat outside an anchored buoy would be much safer.

Phillip - indeed on the PFDs.  That was from about 10 yeas ago at a Beneteau 36.7 North Americans, and I am guilty as charged, since I'm the one in the black jacket.  We were understandably nervous about being on a small boat in the way of 35+, 11,000 lb dump trucks (made by Beneteau).  However, the engine was running, the anchor line was on a slip knot and there was a small buoy attached to the anchor line - we could bail out at a moment's notice and still leave a mark in the water.

An un-anchored pin boat is all but useless for calling OCS boats.  The first question a jury will ask in a redress hearing is, "Were you anchored?"  If not, then the veracity of your testimony will always be questionable.  I have extensive experience in being the pin boat (anchored) and there have been only a couple of instances where collisions occurred - both at a Hobie 16 worlds in Australia.  One was at the finish.

As a committee boat owner I prefer to be an obstruction rather than a target. I thus prefer the windward end rather than the leeward, as in the photo above. Indeed, having experienced a raft of dinghies piled round my anchor cable and along my windward (their leeward) side I now politely decline the leeward end unless I have a well-laid protection mark further up the start line.

As  most sailors prefer to start on starboard tack, and some can be early there can be a rush towards the starboard end of the line with all the inherent risks. If you count the number of times the ODM is hit by comparison to collisions with a starboard end CV (Signal Vessel) you can see why Malcolm prefers the starboard end.

More important than the position of the boats is getting the bias correct. Once sailors are convinced that one end is favoured it is very difficult to get them to change their mind.
I have used a port end committee boat, where we have had multiple fleets starting and use a lapping gate. If the lapping gate is to port and the starting line to starboard there is more chance that, if the lapping boats catch up with a start sequence, they are less likely to have to fight their way through boats approaching their start.
Ideally, everyone should have started by the time the first boat laps but a recall or error is always a possibility.