Note: If as
many people as are researching semiconductors were researching musical
instruments we would be understanding them a lot better. As it is musical
instruments are mysterious to almost magical. (Of course I appreciate any research
on musical instruments!!)
However, the fact that we cannot prove something with mathematical or physical exactness does not necessarily mean that it is not true. I am inclined to believe what is generally believed .
Q: Do your flutes improve with playing in?
Answer: Yes, they certainly
Of course also the player plays 'in' on the instrument. However if I get an
instrument back after a year I myself will
find it playing very significantly better than when it was brand new in my
workshop! The instrument will be more open, easier all over (not just the upper
octave) and warmer. Any good player knows these phenomena.
This issue is related to the next one. Because my instruments become better over time I think: tamper as little as possible with them.
A remark is that instruments do not necessarily become better over hundreds of year if they are very much played.
Q: Should my flutes be rebored after a certain standard period of time?
Let us first do the following thought experiment:
Take one of the famous players who plays on an original 18th century traverso that he loves. Lets call him Mr. K. The flute was made by the famous 18th century maker R. Suppose I had the original reamers still. Now what happens if I propose Mr. K to rebore his treasured original R traverso? I think that he might tell me where to stick my reamers instead!
Answer: No, experience shows that that is not necessary. Professional
players have used my instruments over many years and many concerts showing
convincingly that this is the case.
I stabilise the wood in various ways. The flutes take from four to six months from square blocks of wood to playing instruments. During that process they are at least four times reamed. I have various ways to dry, penetrate with oil and release stress.
Moreover flutes usually go just a little bit un-round and a little bit not straight after a while. Nothing the normal player notices usually. Nevertheless it means that the reamer will take out wood at the wrong place if a flute is rebored after a certain period of playing. This may be alleviated by using short reamers for part of the joint. However this negatively influences the reproducibility aspect. I do use short reamers for some instances though. Boxwood is more prone to this problem than the other woods and should almost never be rebored. If I correct an instrument I measure the bore and make local changes as needed. Only rarely is this done by reboring.
I also think that the bore changes to a certain extent to suite the way the instrument develops.
However if there is any reason for you to think that the flute plays less well or is less well tuned than when you bought it, of course I will check the instrument and take the necessary action (free of charge, of course).
In this connection, a quote from 'Rachel
Brown, The Early Flute'  p38 goes as follows:
"Many makers understandably want to rebore the instrument..... Obviously this is a very personal matter........It is my humble opinion that the bore changes according to the way the instrument is played, and if it is working well it is preferable not to tamper with it."
(I could not have said it better: if the instrument is fine do not tamper!!!!!!!!!!!!!!)
Also in Tromlitz  there is an extensive analysis of the difficulties involved in reboring. Tromlitz indeed supports the idea of reboring but describes in detail the difficulties I am referring to above and the conclusion is that one should (therefore often) replace joints rather than rebore them if necessary. The reason he gives for reboring and is given nowadays is that one 'of course wants the instrument to stay geometrically as it was when new' . This may be an incomplete reasoning. Wooden musical instruments, also Violins for instance, change. It is known (see previous Q) that they often become better. Why, I do not really know!!
I think that my way of stabilizing and a priory reaming a number of times over a longer period of time before leaving my workshop explains the fact that the playing qualities of the flutes stay very satisfactory and improve with playing in.
Again, if the instrument is fine do not tamper.
 Rachel Brown, The
Early Flute. ISBN 0521813913
 J.G.Tromlitz, Ausfuehrlicher und gruendlicher Unterricht die Floete zu spielen, Facsimile Frits Knuf Buren 1973, 1985