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 $38.64   3.497%   $39.9912   $39.99   $40.00   $40.00  
  Purchase #2 $22.55   7.724%   $24.2918   $24.29   $24.30   $24.30  
  Purchase #3 $27.47   7.173%   $29.4404   $29.44   $29.45   $29.40  
  Purchase #4 $82.33   3.821%   $85.4758   $85.48   $85.50   $85.50  
  Purchase #5 $106.30   3.515%   $110.0364   $110.04   $110.05   $110.00  
  Purchase #6 $84.41   11.113%   $93.7905   $93.79   $93.80   $93.80  
  Purchase #7 $150.73   9.646%   $165.2694   $165.27   $165.25   $165.30  
  7 purchase totals = $512.43       $548.2955   $548.30   $548.35   $548.30