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 $28.61   4.368%   $29.8597   $29.86   $29.85   $29.90  
  Purchase #2 $48.78   12.383%   $54.8204   $54.82   $54.80   $54.80  
  Purchase #3 $9.49   7.381%   $10.1905   $10.19   $10.20   $10.20  
  Purchase #4 $90.64   10.857%   $100.4808   $100.48   $100.50   $100.50  
  Purchase #5 $141.58   9.719%   $155.3402   $155.34   $155.35   $155.30  
  Purchase #6 $59.28   10.755%   $65.6556   $65.66   $65.65   $65.70  
  Purchase #7 $156.37   4.298%   $163.0908   $163.09   $163.10   $163.10  
  7 purchase totals = $534.75       $579.4380   $579.44   $579.45   $579.50