Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $36.94   11.844%   $41.3152   $41.32   $41.30   $41.30  
  Purchase #2 $231.47   8.493%   $251.1287   $251.13   $251.15   $251.10  
  Purchase #3 $162.42   4.868%   $170.3266   $170.33   $170.35   $170.30  
  Purchase #4 $55.25   4.564%   $57.7716   $57.77   $57.75   $57.80  
  Purchase #5 $36.77   11.650%   $41.0537   $41.05   $41.05   $41.10  
  Purchase #6 $196.75   13.504%   $223.3191   $223.32   $223.30   $223.30  
  Purchase #7 $242.83   6.141%   $257.7422   $257.74   $257.75   $257.70  
  7 purchase totals = $962.43       $1,042.6571   $1,042.66   $1,042.65   $1,042.60