So is the idea to enter in to the turn slowly enough so you can continuously roll-on?
Yes, that's exactly correct. The right technique being to brake/slow prior to reaching the entry point for the turn, to a speed that is as slow, or slightly slower than the speed you wish to be traveling while in the turn.
The idea being to allow for proper throttle management (being at least on off-idle maintenance roll-on, to minimize the force transferred to the front tire), without the bike exceeding the pre-determined target safe speed at any time while in the corner.
This is true for turns under most all conditions, but of premium importance when taking corners on wet pavement where traction levels are greatly reduced!
The level of importance of slowing an even larger amount below the desired target corner speed, exists when taking
downhill turns. That's because gravity will try to make the bike's speed increase (against the rider's choice) as soon as braking is removed; regardless of throttle position.
For street riding it's way better to slow what ends up being more than was necessary for taking a turn, which results in an even larger safety cushion for applying good throttle roll-on technique in the wet (and/or over tar snakes), than to error in the other direction!
What about decrease radius turns?
The most important aspect for properly adjusting your entry speed for a "decreasing radius" turn, is recognizing that it is this type of corner before getting into it!
Assuming you know in advance (from either looking ahead and reading the corner, or having ridden it before) that a corner is a "decreasing radius", the best technique is to brake sufficiently to slow your speed to slightly BELOW the fastest speed at which the tightest, most challenging, decreasing portion of the turn can be safely taken.
In other words if the constant radius first 1/2 of the corner could be taken safely at 40MPH, but the decreasing radius last 1/3 can only be taken at 30MPH, applying the above approach would consist of consciously reducing your speed down to 30MPH (or preferrably less) prior to getting into the turn. As that it's not a race, nothing lost in going a bit slower in the first portion of the corner than that section could be ridden, for the sake of being properly setup for the more challenging part at the end of the turn.
The objective being for the bike to be traveling at a speed when reaching the decreasing part of the corner where you won't end up being drawn into letting "off" on the throttle at any point.
A bit of roll-off of throttle for a brief moment isn't necessarily the end of the world on a dry day, where tightening the line a little could be done. However finding yourself drawn into coming off the throttle in such a corner on a wet day (which is the theme of this thread) where the pavement is slick, can very likely result in losing front tire grip ..... and a crash! :thumbdown
For corners where you know you'll be crossing tar snakes ahead in the wet; slowing a good 5-10MPH below the speed you wish to be traveling while crossing the snakes while in the turn. is again the pre-approach technique for dealing with that obstacle.
The mantra for the safest technique for setting speed for taking corners in wet and slippery conditions is "slower than normal going in"; allowing for always being able to maintain at least "off-idle maintenance throttle position" (or slight more) from turn entry ...... all the way to exit .... without the bike exceeding the target safe corner speed at any time. :ride
More tips to digest Alex (and anyone else reading that has an interest).