Giving a buck or two to someone who delivers room service falls into a different category for me. It is often not your cabin stewart and it is additional work even if it is. So, I have no problem with it. Besides, if a dollar makes someone happy, it is money well spent. And, it rarely adds up to much, probably the cost of a really cheap t-shirt.
Re. pre-tipping, I do not do that at all, except on when a crew member carries your carry on luggage to the cabin for you. But, again, that is extra service and not anyone you might ever see again. The custom of giving tips after services have been performed is common everywhere. And, as stated, some cultures might consider it an insult to pre-tip. I have found that a friendly attitude and respect will get you the best service possible.