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 $77.30   9.811%   $84.8839   $84.88   $84.90   $84.90  
  Purchase #2 $250.70   8.627%   $272.3279   $272.33   $272.35   $272.30  
  Purchase #3 $22.62   4.157%   $23.5603   $23.56   $23.55   $23.60  
  Purchase #4 $201.29   10.540%   $222.506   $222.51   $222.50   $222.50  
  Purchase #5 $85.43   6.620%   $91.0855   $91.09   $91.10   $91.10  
  Purchase #6 $192.31   4.848%   $201.6332   $201.63   $201.65   $201.60  
  Purchase #7 $84.52   13.266%   $95.7324   $95.73   $95.75   $95.70  
  7 purchase totals = $914.17       $991.7292   $991.73   $991.80   $991.70