Does DNA replication in 5' to 3' (leading strand) need RNA primase?
The DNA polymerase also needs a RNA primer on the leading strand to be able to start polymerization. Afterwards this is not needed anymore, since the replication goes on without a break. On the lagging strand polymerization replication can only work between the replication fork and the next region of double-stranded DNA. See the figure (from here):



The reason for the need for RNA primers is located in the function of the enzymes. While the DNA polymerase can only work on a double stranded template (add nucleotides to the 3'OH-end of the strand) the DNA Primase (actually an RNA polymerase) can work on single stranded targets and thus add the RNA primer there.