Kiltmadoc - You're quite welcome! Yes, you may leave those sheaves in place. If you're ever replacing something at your masthead, take a look at them and see if you can't take some replacements with you - That small groove does pinch rope and will eventually break internal fibers. It's not quite as severe a problem as a V-Shape but it will eventually destroy your halyard by pinching the core fibers at the edge of the inner groove. The good news is that knowing you have those sheaves, you can easily find replacements. You'll know when this is happening because your halyard will develop soft spots and bumps: Replace any line that's like that immediately - It's about to fail!
zz4gta's advice is sound, except please don't ever use Sta-Set for halyards. It's a relatively medium-stretch line with ~3.6% elongation at 30% yield stress (1500# for 3/8", or just 500# stress on the line, which, with a 42:1 winch, requires less than 12# force on your winch handle!), designed to be resilient with shock loading, as is needed in sheets. You will have never-ending halyard sag with Sta-Set. That's because 30% of yield stress is really too much of a load for it: Typically, an 8:1 safety factor is needed. Warpspeed, by comparison, has .79% stretch at 30% yield and that's >>3900#, requiring 93# force on that #42 winch - So you can see why it's appropriate: You shouldn't be able to crank so much force on the line that you endanger it. The reason the Sta-Set keeps creeping as a halyard is that you're actually exceeding its yield stress limit over time. It will harden but a hardened rope has lost much of its strength - Perhaps 70% is gone. It's almost impossible to tell because at this stage, the rope is trashed. Unfortunately, many sail with ropes that are in this stage and the way to tell if they are hardened is to feel - They will feel stiff and crackle when bent. Replace any ropes that feel this way - They are in eminent danger of failure and because they have no elongation left, will fail suddenly (Without warning). Never, ever, use ropes that are stiff or have soft spots or bulges to climb a mast!
I'm a shipwright, rigger and professional sailor, in business as My Boat Works, in Los Angeles. I also make super quality, High-Definition instructional boating movies! I'm here to give my expertise to boaters, as I was helped. It's my pleasure to help you enjoy activities I love so. Cheers! - Jerr