1. We have to group in such a that the max. number of Boolean variables should have both 0 and 1 combinations. So that we can minimize that variable in the expression.

2. At least one minterm should be in the group. (Focus on minterms, not on don't cares.)

3. If some of don't cares are not coming into the groups, no need to bother about it.

Don't care conditions should be exploited for our maximum benefit. 

If a certain don't care case is helping us to make a larger pair, then include it otherwise we should be happy to exclude them. 

The rule in Boolean minimization using Karnaugh map is to choose among several possible groupings the one which will give the fewest number of groups but having the largest possible size. Fewer number of groups means fewer number of product terms and larger size of groups means fewer number of literals appearing in a product term. Generally, there may be several equivalent but non-identical minimized Boolean expressions that can result from a given minimization process.

Suppose i do minimization interms of SOP and POS seperately, any one be less literals. Shall i choose any one?

Don't care cases would be happy to merge with SOP or POS groups and they won't feel bad if none of them include in their group. 

I suggest you to read Brayton, Robert King; Hachtel, Gary D.; McMullen, Curtis T.; Sangiovanni-Vincentelli, Alberto L.. (1984), Logic Minimization Algorithms for VLSI Synthesis, Kluwer Academic Publishers, ISBN 0-89838-164-9. This book is the holy grail of the two-level minimization. It describes each step comprissing the ESPRESSO tool. You can find a summary of this tool in the Wikipedia. A source code (in C) is also provided here: https://github.com/nelsonjchen/espresso-logic-minimizer.

https://en.wikipedia.org/wiki/Espresso_heuristic_logic_minimizer

Thanks for valuable information...