I would center the plate over the nighstands, I think.

What I do for fixtures where I don't have an exact position is ask the electrician to leave a long loop of wire in the wall and then have it poking out of a hole in the approximate location.

Then when dimensions are settled I have them come back and install a box. It works better if you know one of the dimensions, because you can have a nailer to attach the box to at the vertical or horizontal position that is long enough to attach the box at various positions.

I remember a ways back when bronwynsmom was here, she recommended that a swing arm lamp should be placed in the same way as an end table lamp would be...

[Contemporary Bedroom[(https://www.houzz.com/photos/contemporary-bedroom-ideas-phbr1-bp~t_715~s_2103) by Toronto Media & Bloggers Jennifer - Rambling Renovators

[Eclectic Bedroom[(https://www.houzz.com/photos/eclectic-bedroom-ideas-phbr1-bp~t_715~s_2104) by Venice Interior Designers & Decorators Katerina Tana Design

[Traditional Bedroom[(https://www.houzz.com/photos/traditional-bedroom-ideas-phbr1-bp~t_715~s_2107) by Los Angeles Interior Designers & Decorators Annette English

[Contemporary Bedroom[(https://www.houzz.com/photos/contemporary-bedroom-ideas-phbr1-bp~t_715~s_2103) by New York Architects & Building Designers Workshop/apd

I think it depends on the length of the arm and whether you plan on using to read in bed. Some of these pictured would NOT work for reading, so if you want them placed over the table and want to read in bed, make sure you can get a light that will have a long enough arm to work.

This is hung closer to the bed, but the size and night stand arrangement works for that and you could easily read with it

This is centered but long enough to reach the side of the bed for reading

Annie, I love how the wiring is hidden under the molding in the first photo.

I,opted to put two small spot lights over my bed. I love them for reading! Also less clutter than wall lamps.