This is a complex question. There is a wide variety of baseboard types as well as several ways to make hot water in boilers today. the answer was simpler 20 years ago. This could be an interesting question because many recommendations can be made by different people.
Some people in companies are recommending that you not make hot water with a tankless coil boiler. A high quality mixing valve can be adjusted from high boiler temperatures down to safe water temperatures, others can not be depended upon to remain steady and safe over time. Few actually warrantee the mixing valve to be scald-proof. Generally most people will not be damaged by water temperatures below 110F. Japanese like to sit in hot tubs at 113F, but local codes may require the tub temperature to be below 102F.
Cast-iron baseboard is often rated at 200F as a stated btuh output. Copper-tube baseboard is often rated at 180F. But water temperatures above and below these can be used as desired. For instance, because of the increased amount of insulation used today, wall-to-wall cast-iron or finned-tube baseboard would need only to reach 120F to satisfy the heat lost from an average living room. Radiant floor temperatures need rarely exceed 100F.
The old classic settings for a tankless-coil boiler were often 210F high limit, 180F low-limit with a 15F differential. There was a direct line from the tankless coil to the dishwasher to produce fat-rinsing temperatures, with a 140F mixing valve setting. As water heaters are often set at 120F today, the 140F setting of the mixing valve may be too high for some code areas because a scald can happen even lower than that temperature. It makes little sense to have an anti-scald rated mixing valve produce scalding water. So this is an area of controversy that is still unsettled between government, manufacturer, and customer.