Cross chaining and chain angle
 

I use a triple and I was wondering if I should avoid using the highest and lowest cogs while I'm in the middle chainring? If I'm in the largest or lowest chainring, I know I should stay with the highest and lowest cogs, respectively, to keep the chain more or less straight.

But is chain angle that much of an issue as long as I'm in the middle chainring?

As long as the chain is not rubbing the front der. it's ok.

Two considerations.

the first is that the chain clears the FD cage if it can't be trimmed, because you don't want to saw it through. Also that it clears the outer chainring coming from the outermost rear sprocket. That takes care of the minimum practicalities. Now the cross chaining issue.

While many people talk about cross chaining in a digital sense as in non-crossed combinations are OK and crossed aren't, thatosn't how chains see it. Chains run best when they're aligned, and wear faster and are less efficient when they aren't, with the degree of the problem related to the amount of deflection. It's a spectrum, with minor angles being minor problems, and more being worse.

So coming to your question, the inner and outer sprockets are the worst to use with the middle ring, but 2nd ones on either side are only slightly better, and so on until you get the middle and middle. So try to ride with the more aligned groups, but if it's the right gear for the moment using the outermost with the middle from time to time won't be a disaster.