Chances are indexes will become fragmented to some degree but it will depend on things like the index fill factor, which columns you are updating, the columns and order of the indexes, lots of things, as to whether they will or how much they will become fragmented.

For example if you insert a ton of data into a table that only has a clustered index where the leading edge of the index is an identity column then you won't see much fragmentation in that clustered index, however nonclustered indexes could be affected. Or let's say you update many rows in the table, but you only update an integer column that was present when all rows in the table were first inserted, then you will also not see much fragmentation because integer is a fixed-width data type.

But let's say you insert many rows into a table where the clustered index is on last name, if data being imported and data in the table are both distributed in a similar way then chances of encountering page splits and fragmentation ensuing is high. Or say you will be updating many rows, updating a varchar column that was previously null but now will have a value, and the fill factor is 100, chances of fragmentation are high.

Even deleting can cause fragmentation, so be aware of that possibility as well.

The good news is that index fragmentation is quite easy to solve. Check the scripts at http://ola.hallengren.com for a solution that will work for many systems with no special effort other than compiling some stored procedures and functions and setting up a scheduled job, and will be mostly hands off if even only a small nightly maintenance windows exists for the database.