So long as it starts right up (less than one engine revolution). If you actually have to let it crank for more than a couple (3) seconds, you are working the starter harder than you need to. The glowplugs make starting much, much easier on the starter and battery. I assume you have the AC on all the time, so you are putting a pretty good load on the alternator, too.

It only takes a few seconds at that temp to get the plugs hot, so I always use them.

Believe me, in the winter you will want to use them anyway!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!