Theme: No Middle Ground - The unifier (37Across) says ON and OFF can be placed in front of each of the other two-word entries. This will take some explaining.
17A. Where sea meets sand : SHORELINE. Not necessarily sand, NH has only 14 miles. Onshore: either a wind blowing towards the shore or something already on the shore. Offshore: the opposite except things offshore be much farther away. On-line and off-line are usually thought of these days as Internet connections.
25A. Behind-the-scenes worker : STAGEHAND. Workers associated with a stage, roadies are also behind the stage workers but not in one spot. Onstage and offstage are just that. On hand and offhand are not just opposites. Onhand means in one's possession, while offhand means in an informal or casual manner.
37A. Light controller—either of its first two words can precede either part of 17-, 25-, 51- and 61-Across : ON/OFF SWITCH
51A. Distract : SIDETRACK. Originally referred to trains. For the most part, onside and offside are sports-related terms. On track is keeping to a schedule and off track is more like something that isn't directly associated with the matter at hand.
61A. Sentry's job : GUARD DUTY. To many these days, Guard duty involves being shipped overseas. Being on guard will prevent you from being caught off guard. On duty and off duty are just that.
Argyle here. It seems funny to have a Jerome puzzle without puns. Well, I may have wasted too much time on the theme; I better get busy on the rest of it.
Across:
1. Poker Flat chronicler Harte : BRET, His short story, "The Outcasts of Poker Flat" was first published in January 1869 in the magazine Overland Monthly
5. Syrup brand : KARO. Light corn syrup.
9. Scatter : STREW
14. Plane opening? : AERO. Aeroplane.
15. Farsi-speaking republic : IRAN. Farsi 1878, modern Persian language, the usual Iranian word for it, from Fars, Arabic name for region of Pars (no "p" in Arabic) in Iran.
16. Sports venue : ARENA
19. Like most attics : DUSTY
20. Mob enforcer : HITMAN
21. Gp. concerned with fluoride safety : ADA. American Dental Association.
23. Links elevator? : TEE. Golf.
24. Old Great Lakes natives : ERIEs
28. Christmas mo. : DEC.
29. Water temperature gauge? : TOE
31. Pro vote : YEA
32. USPS carrier's assignment : RTE.
33. Words of sympathy : "I CARE"
35. Potato cutter : RICER. Has anyone bought one since the last time we had this item.
40. Flora eaters, perhaps : FAUNA. Occasionally, it works the other way. Venus Flytrap.
42. Brief and forceful : PITHY
43. Pilot's no. : ALT. Altitude.
44. Toothed tool : SAW
47. Unused : NEW
48. Rock guitarist's aid : AMP
54. Spring time : APRIL
56. Place for a pint : BAR
57. Place for a cup : BRA. (hee, hee, hee)
58. Anatomical ring : AREOLA. (hee, hee, hee)
59. Steppes native : TATAR. Good map of the steppes
63. Carrying a lot of weight : OBESE. One of the nicer ways to say 'fat'.
64. Cold capital? : OSLO. Norway.
65. Largest continent : ASIA
66. Used hip boots : WADED
67. Feat : DEED
68. Winemaking waste : LEES. The dregs.
Down:
1. Lambasted : BASHED
2. Put to work again : REHIRE
3. Titillating : EROTIC
4. Singer with the Mel-Tones : TORMÉ. Some good info with this mellow number. Willow Road(3:01) from 1946.
5. Brick baker : KILN
6. George W.'s first press secretary : ARI. Ari Fleischer.
7. Attacked with clubs and such : RAN AT
8. In the future : ONE DAY
9. Glum : SAD
10. Liar's undoing : TRUTH
11. Fact-finding process : RESEARCH
12. Understanding between nations : ENTENTE. From French, as you might expect.
13. Method : WAY
18. It stretches from Maine to Florida : EAST COAST
22. Make better, as cheddar : AGE
25. Lord's laborer : SERF. Lord of the manor, that is.
26. Falling object's direction : EARTHWARD
27. __ Spiegel: German magazine : DER. Europe's leading news magazine.
30. Stumblebum : OAF
33. Roadside rest stop : INN
34. Clairvoyance, briefly : ESP
36. Like many a slick road : ICY
37. Passé : OUTDATED
38. Lash flash? : WINK. Good one.
39. Suffix with cord : ITE. A smokeless explosive powder consisting of nitrocellulose, nitroglycerin, and petrolatum that has been dissolved in acetone, dried, and extruded in cords.
40. Scale fourths : FAs. JzB?
41. Fictional Arabic woodcutter : ALI BABA
45. Wall St. hedger : ARB.. Arbitrageur.
46. Ares or Mars : WAR GOD. Greek or Roman.
48. Stimulate : AROUSE
49. Uncle __: Berle nickname : MILTIE
50. Western dry lakes : PLAYAs. Pictures.
52. How to turn something into nothing? : ERASE
53. Effect's partner : CAUSE
55. Go by bike : PEDAL
58. Youngest to reach 500 HRs : A-ROD. Third baseman for the New York Yankees.
59. Auto club offering : TOW
60. What mad people see? : RED
62. Pint contents : ALE
Argyle
