I think as Matt says, consistency is everything. Whatever processing steps you undertake, unless you really do make an obvious error, keep doing the same thing.
You may wish to add a 2% stop bath immediately after you have poured the developer out. This does two things, first and foremost, it immediately stops development dead instantly. Secondly, it will certainly reduce the amount of developer residue getting into the bleach, thereby making your bleach last longer. Another reason is that you can then relax and take your time and arrange things before your pour the bleach in. I myself use a 30 second stop bath and have done so for probably the last 25 years of C41 development.
Bleach is the most expensive part of the process, bleach has about double the processing capacity; meaning you can save a reasonable amount of money in the future if you get further into colour negative processing and start mixing much of your own chemistry from raw chemicals.
For colour processing, pretty much nothing is better than a Jobo rotary processor. If you do go down that route, then if you can, get a Jobo processor with a lift already attached. Having a lift allows you to time baths to within 1 second, day in day out.
The 1540 tank is probably the best tank for economy and efficiency. This tank, which will develop 4 rolls of 35mm or 4 rolls of 120 at a time effectively requires 500ml of chemistry. With Kodak, many moons ago giving their C41 process as 8 rolls of 135 film per litre, works in perfectly as you'll use exactly 1 litre of C41 chemistry for 8 rolls using the 1540 tank.
The 1540 tank is a single 1510 tank + the 1530 extension. The other tank that is handy to have is the 1520 tank, which, as the name implies, holds 2x rolls of 35mm film. You can also use the small 2500 tank to develop 4x5" sheet film, if your photography one day heads in that direction.
As far as I know, C41 process is 37.77ºC carried or as often stated 37.8ºC which effectively equates to 100ºF. Developing time is strictly 3'15", if you wish to push the film one stop, you add 30" which means that not stopping your development exactly at 3'15 but effectively stopping it at 3'30" you have pushed the film ½ a stop. C41 is the shortest standard film process I know of, so getting developing times right, or at least consistent for your first bath, is imperative.
When you push C41 you will find your mid-tones pretty much stay the same, however your highlights (white stuff in particular) will be brighter and maybe losing fine detail, while your shadows will be darker, tending to black earlier. So for some subjects, especially portraiture, you usually need to be careful about developing times in the first C41 bath.
As for agitation, we used nitrogen bursts in a deep tank processor, these gave a burst of gas every 8 seconds for virtually continuous solution movement to ensure very even and consistent development of the negatives. Rotary processing in a Jobo processor, gives constant agitation. I have processed C41 test strips in my home Jobo processsor which were virtually identical to our lab in their super controlled processing baths. What I'm trying to say, is that for very even development of C41 film, an almost constant agitation of the solution is where it is at. It does not need to be violent, gentle but constant agitation is your friend. In general, if you rotate a tank for agitation, then the chemistry will probably be flowing around for about 10-15 seconds after you place it back down. At around this time limit, the flow of fresh chemistry over the film pretty much stops, so another inversion or agitation movement is required. Remember this is an extremely short process, one cannot work on the basis that a longer processing time; which is standard for almost all other B&W, will even out any irregularities incurred due to lack of agitation.
My Jobo processor was the CPE2 with lift. I bought it new in the late 80's and processed literally thousands of rolls and sheet film in it. It died a couple of years ago after around 30 years of constant film processing, so I now develop once again by hand. But I am using a Jobo tank roller sitting in a tray of heated water and rotating the tank by hand, for my now minimal C41 film throughput; works a treat.
Mick.
