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 $26.43   9.274%   $28.8811   $28.88   $28.90   $28.90  
  Purchase #2 $77.60   12.599%   $87.3768   $87.38   $87.40   $87.40  
  Purchase #3 $58.82   3.192%   $60.6975   $60.70   $60.70   $60.70  
  Purchase #4 $186.62   7.702%   $200.9935   $200.99   $201.00   $201.00  
  Purchase #5 $69.44   6.672%   $74.073   $74.07   $74.05   $74.10  
  Purchase #6 $38.90   12.143%   $43.6236   $43.62   $43.60   $43.60  
  Purchase #7 $135.96   7.658%   $146.3718   $146.37   $146.35   $146.40  
  7 purchase totals = $593.77       $642.0173   $642.01   $642.00   $642.10