If they are matched brand across the axle you'll be fine for the sort of driving you describe - assuming the new rears are of comparable quality to the fronts. If you put ditchfinders on the rear you might find it a bit unpleasant in the wet!
Regarding sizes - the OP mused in the comments about putting 225/50/R16 on the rear with 205/50/R16 on the front.
The suggested change actually provides a 2cm difference (it's 1cm in radius so double it for the diameter) which doesn't sound a great deal but it can potentially affect the vehicle's rake in a way not anticipated by the aerodynamics designers (which could be a big deal at high speeds), while an increased rake angle isn't a de facto bad thing (and in many cases can be quite positive) without seeing the changes to airflow (such as in a wind tunnel or CFD) you can't predict what will happen. The flow over both the top of the car and beneath will change and even small changes in angle can lead to quite dramatic differences.
this then also affects a) the spring rate (you've got 2cm more of sidewall), b) the circumference changes by over 6 cm so the revolutions per mile shifts by over thirty - so you are getting significantly difference wheelspeeds between the front and rear axles.
The ride height of a normal road car is expected to vary - it changes as the weight in the back changes (passengers luggage etc) but this is in the opposite direction - and these scenarios will have been accounted for by the designers of the car. It's difficult to create a scenario where the ride height at the rear will actually increase beyond the original design specs through normal use.
Of course it could be fine and you would never have a problem - personally it's not a risk I would take.
The taller tires on the rear also affects a) the spring rate (you've got 2cm more of sidewall), and b) the circumference changes by over 6 cm so the revolutions per mile shifts by over thirty - so you are getting significantly different wheelspeeds between the front and rear axles, while an FWD car doesn't have to worry about this causing wind-up in the center differential like a 4WD does this change in wheel speed is going to make any stability control, traction control and ABS systems potentially very confused.
The increased height could potentially change the angles for the rear control arms (depends on suspension layout) affecting camber at the rear giving negative handling affects and/or uneven tire wear.
Basically don't use mismatched sizes unless the car is designed for it (such as RWD BMWs, Mercedes' etc)!