In real life, I've been raised to tip certain amounts for different services. If I go out to eat, have some item delivered specially (food, newspaper, etc.), or have a hotel room cleaned, I plan to be tipping those providing the service. There are certain amounts for each type of service, and I was taught about it growing up. There's usually not someone to teach you in SL about tipping etiquette.
First of all, a tip of any amount is usually going to be appreciated. If you want to tip someone but only have 5L and are concerned they may be offended, send the tip then let the person know how much you appreciate what they do to make your SL experience better. It will ensure they know that you value them. The kind words will make the tip that much greater.
At clubs, there is the venue, DJs, hosts, and potentially dancers. The venue needs money in order to survive and many rely on tips. If you really enjoy visiting a club frequently, consider donating to the club itself to help them cover their rent. If the club has dancers with tip jars, and they are making your experience there better, consider giving a tip. Hosts work hard to keep the party going. They have to advertise, welcome people, and interact consistently to keep the party interesting and inviting. DJs play music for you! They have costs, such as the cost of the music they play, and potential equipment they may use, as well as the cost to stream the music to you. If you like the music, tip the DJ.
If you enjoy going to live events, there is an entertainer, the venue, and potentially a host. Many venues pay the entertainer before hand. Some entertainers give the venue some percentage of their tips while others do not. If you enjoy the performance, it is nice to tip both the venue and the performer. If the venue does not have money, they will not be able to bring the performer back or invite other people to perform. If there is a host advertising, welcoming people, and socializing to make your experience better, consider tipping as well, especially if it is because of this person that you heard about the event.
There may be a place you visit frequently. There may not be any events, but it is a place you enjoy spending your time in SL. Land costs money in SL. There is a monthly fee from LL depending on the size of the land. Some people rent from others and pay a weekly fee, again dependent upon the size of the land and how much they can place on their land. Quite a few places leave tip jars or objects out so you can tip. It may be in a wishing well, animal, some form of nature, heart, or some other item - often with text above it to ask for tips.
You do not need to worry about tipping a person or place every time you visit, but if you have a good experience, visit a place a lot, go to a lot of parties with the same host/DJ, then consider tipping every once in a while if you can. There are even other situations I did not really dive into where a tip would be appreciated. One example would be for auctioneers. Not everyone can tip, and that's okay. Some people can't tip very much, and that's okay, too. If you can't tip, or you have a small amount to tip, please at least send a message of encouragement to the entertainer or venue owner to let them know how great of an experience they have given you in SL.
On a slightly unrelated note, I am reminded by this article that I stayed at a hotel with some family over the summer for a reunion. I was temporarily babysitting my young cousin, and he hears someone knocking on doors and asks who it is. I let him know about house keeping. When they came closer and knocked across the hall, he said in terror, "Oh no, it's the house creepers!" Haha.
