Chandana,
IMO for most sites the meat description is one of the most critical elements in On-page optimization (notice I did not say on-site). When we query a search engine for a product, service, or information and a SERP is returned, for organic search we look at the title and meta description to determine which is either the first to appear to have an answer or the most likely answer to our query.
While many CMS plug-ins have an option for a "site-wide" meta description, that is not the best option on sites with more than a couple of pages. You want a meta description that states: "Here is the best answer to what you searched for: Get it now! Get it Cheap!" (obviously, in a way that reflects your page.) Without showing you have the answer to the query you are going to get less click throughs and therefore traffic to your site.
Even if CTR is not a part of the algorithm, you will not have a chance to sell, etc those who are never getting to your site.
Now, in using the first 155 words you are likely losing an opportunity to sell the overall of the page while making it a bit easier on filling out all that is needed for a page in search. The first few sentences in a paragraph generally are broad and not necessarily speaking to the "answer is here, answer is now, answer is cheapest!" So, we do a lot of web pages and, typically, if I have a complaint with a site on final review, most will be around meta descriptions that fail to show this is where the answer is to the searcher's query.
Hope this provides a bit of assistance,
Robert