As you ask "without a doubt" then the answer must be "No, not always, but often".
I am constantly being informed by the experts who actually make whisky that 80% to 90% of the end flavour of any whisky comes from the cask.
But, with some whiskies, prolonged maturation in oak can be just too much, creating an end flavour which can be perceived to be not as "good" as younger expressions.
I was in a presentation with someone from Laphroaig who explained the effect of longer maturation very well with respect to Laphroaig whisky.
Laphroaig is renowned most of all for the 10y/o expression. This is a unique taste, very medicinal with a mixture of peat, iodine, seaweed ..etc. A flavour we all know.
If you look at the older 15y/o expression, it is much more rounded with less "in yer face" character and flavours. All in all, a much smoother dram.
Take this concept further and look at the 30y/o:
A very rounded dram which is very different to the younger expressions.
This is illustrated by what some forum members say of Laphroaig 30:
"Extreme good balance between sherry, oak and peat" (from Tom).
"It has a good deal of fruit and sherry in it, a nice dose of oak, and just a small note of traditional Laphroaig character in the background. It has a very rounded flavor & is quite richly flavored as well." (Choochoo).
The longer the maturation period, the more interaction with the oak, therefore the oak characteristics tend to prevail much more.
With some whiskies, this will create a different and probably lesser liked flavour than a younger expression, but once again, this can also be a matter of personal taste and what 'you' expect from a certain dram.
(Conversely, some whiskies require longer in a cask and can be bottled at too young an age!)
Finally, to stay on the topic of Laphroaig, I will look at the very well-liked QC.
This is a younger Laphroaig, aged for around 7-8 years in normal (500 litre) casks and then switched in quarter casks (of 125 litres) for less than a year - around 8 months.
The use of a QC means that a larger percentage of the content is in contact with the wood which in turn, means that the effect is to produce a whisky older in character than its years.
Most people, upon drinking QC for the first time, place this as being a more mature or older dram than it really is.
This shows the effect of the wood on the whisky's maturation and ageing process.
(Sorry for the long post)