It's all about how the piece ends up sounding -- and not about how YOU end up sounding ("Boy, listen to that tuba!").
I would not, in general, do this. The best assumption is that the tuba part has been orchestrated to sound as the composer intends, and so should be played that way. So unless a conductor specifically asks that a piece be played down an octave (or more frequently/realistically, that a particular note or passage be played in this way), then play it as written.
That being said, there are circumstances in which that assumption may be false and it can work well to play the part down an octave. This can occur, for example, when your band is playing an arrangement intended primarily for middle school students -- and so the range required by the piece has been adjusted accordingly. But even then, care must be taken -- because not only has this been done for the tuba part, but it's been done for all the parts. And having the tuba part deviate can result in a peculiar effect.
The tuba should not draw attention to itself -- except in passages where that's the clear intent of the composer/arranger.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)