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 $81.53   10.385%   $89.9969   $90.00   $90.00   $90.00  
  Purchase #2 $43.58   3.379%   $45.0526   $45.05   $45.05   $45.10  
  Purchase #3 $88.89   11.106%   $98.7621   $98.76   $98.75   $98.80  
  Purchase #4 $100.66   7.152%   $107.8592   $107.86   $107.85   $107.90  
  Purchase #5 $56.95   5.685%   $60.1876   $60.19   $60.20   $60.20  
  Purchase #6 $72.49   4.233%   $75.5585   $75.56   $75.55   $75.60  
  Purchase #7 $55.25   8.174%   $59.7661   $59.77   $59.75   $59.80  
  7 purchase totals = $499.35       $537.1830   $537.19   $537.15   $537.40