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 $16.79   11.488%   $18.7188   $18.72   $18.70   $18.70  
  Purchase #2 $27.44   3.866%   $28.5008   $28.50   $28.50   $28.50  
  Purchase #3 $12.72   3.740%   $13.1957   $13.20   $13.20   $13.20  
  Purchase #4 $134.33   11.663%   $149.9969   $150.00   $150.00   $150.00  
  Purchase #5 $23.64   5.847%   $25.0222   $25.02   $25.00   $25.00  
  Purchase #6 $122.63   12.763%   $138.2813   $138.28   $138.30   $138.30  
  Purchase #7 $168.38   11.654%   $188.003   $188.00   $188.00   $188.00  
  7 purchase totals = $505.93       $561.7187   $561.72   $561.70   $561.70