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 $79.40   12.445%   $89.2813   $89.28   $89.30   $89.30  
  Purchase #2 $201.85   13.839%   $229.784   $229.78   $229.80   $229.80  
  Purchase #3 $22.30   3.332%   $23.043   $23.04   $23.05   $23.00  
  Purchase #4 $222.76   3.237%   $229.9707   $229.97   $229.95   $230.00  
  Purchase #5 $12.81   3.249%   $13.2262   $13.23   $13.25   $13.20  
  Purchase #6 $222.98   6.796%   $238.1337   $238.13   $238.15   $238.10  
  Purchase #7 $249.92   13.866%   $284.5739   $284.57   $284.55   $284.60  
  7 purchase totals = $1,012.02       $1,108.0128   $1,108.00   $1,108.05   $1,108.00