For most private flight, it really doesn't matter. Even in the States, where high-level cruising is more common.
When I was in the States I was flying a Super Cub, often at 8,000-10,000', and I always used 85kt (100mph) as my "TAS". What I found in practice was that once I got up to a few thousand feet, each leg would take a little less time than I'd planned. So my in-flight thoughts would be "I planned on being over this mountain at 43 minutes, it's now 39 minutes and I've reached the mountain a little early, as expected because my TAS is slightly low." Simple, huh?
Then, as you climb further, you find that the engine starts loosing power (assuming a normally-aspirated engine). So your IAS starts dropping off. I found that at 10,000' and with the throttle wide open, I'd have an IAS of around 85-90mph, and a TAS of around 105-110mph - still close enough to the planned 100mph for my purposes.
It's worth knowing how to find the TAS. It's even worth calculating the TAS for the kind of temperatures and altutides you're likely to use, just to see how different to the IAS it is. Once you know the ballpark figures, it's up to you whether you use them or not, but personally I wouldn't bother.
Just my opinion, of course. And I'm sure I'll be expected to calculate the TAS when I do my CPL, so I don't claim that what I'm suggesting is actually "correct", just convenient!
FFF
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