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 $8.51   10.633%   $9.4149   $9.41   $9.40   $9.40  
  Purchase #2 $140.78   5.191%   $148.0879   $148.09   $148.10   $148.10  
  Purchase #3 $27.82   5.268%   $29.2856   $29.29   $29.30   $29.30  
  Purchase #4 $189.44   5.865%   $200.5507   $200.55   $200.55   $200.60  
  Purchase #5 $9.38   6.567%   $9.996   $10.00   $10.00   $10.00  
  Purchase #6 $115.82   7.752%   $124.7984   $124.80   $124.80   $124.80  
  Purchase #7 $109.98   7.395%   $118.113   $118.11   $118.10   $118.10  
  7 purchase totals = $601.73       $640.2465   $640.25   $640.25   $640.30