Shy...
Quite right, outside Class ABCD you may request a RIS or a RAS, it's up to you to ask irrespective of your flight rules (IFR or VFR) and flight conditions (IMC or VMC). Provision of that service is down to the ATCU concerned. They may be able to help you, they may not. It depends on a lot of factors (controller workload, your alt, radar coverage, are a few examples). Don't be afraid to ask.
Inside class D (or A/B/C) you don't get a choice. You are either placed under a Radar Control Service or you aren't. It's not a question of accepting the service or asking for it. Now if you are operating under VFR and have been given a crossing clearance based on that fact, the controller concerned might ASK if you can accept radar vectors if neccesary, AND MAINTAIN VMC. If you can't he will have to think of another plan.
It's entirely possible to cross class D and NOT receive a Radar Control Service. You would be in receipt of a "procedural" crossing clearance and would be under an Approach Control Service. Given that you are VFR, its doesn't mean a whole lot, as the controller concerned isn't responsible for your separation from everyone else, you are.
See, can of worms isn't it? Throw in "Duty of Care" and you will see why some ATCOs have serious problems with VFR ops in class D airspace.
Rgds BEX