I was at my home parish this Sunday and, just like last year, the priest has decided that throughout all of Lent our crucifix will be covered. :(
Long story short: It's both incorrect and spiritually inappropriate before Passiontide (at the earliest) or Holy Thursday.
I can quote to you the GIRM, Paschale Solemnitatis, the CCCB (my episcopal conference), the USCCB, and EWTN.
When I have to deal with a covered crucifix throughout all of Lent, I believe that it seriously damages my experience at Mass. The worship itself seems directionless. While the tabernacle is in an appropriate place, it is unadorned, and the crucifix should act as a more visual reminder of Christ's presence.
I believe it really changes the tone of Mass. At least it does for me. Lex Orandi Lex Credendi.
I believe that the covering of the crucifix also adds, intentionally or not, to the common theme these days which makes the priest the centre of the Mass. This is most clear through the common versus orientem posture of the priest that has become the de facto norm, as well as the situation of priests putting their own personal touches to the text of the Mass. By removing the crucifix, my priest seems to be directing attention away from that part of the Sanctuary, and towards himself. It becomes "his Mass", no longer the Holy Sacrifice of the Mass.
In my perfect parish, the Altar, tabernacle, and crucifix would all be placed in the same part of the Sanctuary, but certainly not one of these elements can be removed without serious damage to the Mass. The decision to move tabernacles to side chapels is just as damaging, and just as obviously harmful to us, the faithful. We need to be able to focus our devotion on Christ, and those three elements both work together to help us do so visually. The crucifix reminds us of His crucifixion, the tabernacle of His True Present, and the Altar of His sacrifice of Himself. Lacking one of these elements, we are all impoverished.