I wouldn't risk it in March as you can get some frosty nights which will affect all tender seedlings. Methinks you are a bit too soon with starting sowing this year's annuals. If you sow in late March then they will get away just as well if not better as things are warming up and you can then put them in your cold frame to harden off without the risk. I would wait anyway until they have been transplanted into a larger pot or cell and are much more well-established with several leaves on a bushy plant. When they have filled their allotted space and the weather is more benign, then put them in the frame but be prepared to cover it with a fleece or something if the night temperature dips.
