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 $20.43   9.832%   $22.4387   $22.44   $22.45   $22.40  
  Purchase #2 $78.93   12.553%   $88.8381   $88.84   $88.85   $88.80  
  Purchase #3 $231.45   4.508%   $241.8838   $241.88   $241.90   $241.90  
  Purchase #4 $14.38   13.114%   $16.2658   $16.27   $16.25   $16.30  
  Purchase #5 $10.38   6.245%   $11.0282   $11.03   $11.05   $11.00  
  Purchase #6 $30.29   5.162%   $31.8536   $31.85   $31.85   $31.90  
  Purchase #7 $156.75   6.794%   $167.3996   $167.40   $167.40   $167.40  
  7 purchase totals = $542.61       $579.7078   $579.71   $579.75   $579.70