As an U.S. American having lived many years in Germany, Netherlands, and now Belgium, my experience is that servers appreciate tips very much regardless of whether it's a rounded number or not. Here's why:

• If paying a bill for, say, EUR 27,19, with two EUR 20 bills, I'll tell the server to give me 10 back. So = 2,81 tip.
• If paying same bill but I've received exceptional service, I'll tell the server to give me 8 back. So, 4,81 tip.
• If paying same bill with VISA, I might tell the server to make it 30 (before s/he enters the amount in the card machine). So = 2,81 tip.
• If paying same bill with VISA, I might tell the server to make it 30,19. So = 3,00 tip.

NOTE: I would never pay a server with two 20s on a 27,19 bill and tell her to give me 9,81 back so that s/he could have a round number, 3,00 tip. The server would in fact wonder why I didn't ask for 10 back, since s/he would not have had the hassle of making and counting out change. True, I could tell the server, "Give me the change but keep 3,00 for yourself," but then s/he's still in the counting-out-change mode. Why not round up to the nearest round number to take the burden of making and counting change off of the server? If everyone does this then, at the end of the day, she'll count up her loose change but once and, voila, keep it!

NOTE: If paying with VISA on a 27,19 bill, I MIGHT tell the server to make it 32,19, so that s/he can have a round number, 5,00 tip. That would be an almost 20% tip, and a round number, and s/he would be very happy with it. But, I dare say, even if I told her to charge 33,00 to the VISA card, resulting in an uneven number tip of 5,81, s/he'd still be quite happy.

ANECDOTE: I was eating lunch with a younger colleague--a native Belgian--this past Monday and he took the bill. It came to 21,30. He gave the server a 20 and a 5 and told her he did not need any change in return. So - 3,70 tip. She did not look displeased in the least.

As you can see, there's a lot of nuance. As a result, there is no need to feel an allegiance to any imagined protocol, including any protocol based on the server's (imagined) preferences. One of the things I like about tipping in Europe is that so many servers show genuine appreciation and offer genuine thanks for a tip that's "merely" 10-15 percent of the bill.