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 $29.38   12.500%   $33.0525   $33.05   $33.05   $33.10  
  Purchase #2 $9.34   7.135%   $10.0064   $10.01   $10.00   $10.00  
  Purchase #3 $51.74   4.262%   $53.9452   $53.95   $53.95   $53.90  
  Purchase #4 $32.41   8.483%   $35.1593   $35.16   $35.15   $35.20  
  Purchase #5 $159.28   13.106%   $180.1552   $180.16   $180.15   $180.20  
  Purchase #6 $180.87   9.185%   $197.4829   $197.48   $197.50   $197.50  
  Purchase #7 $37.27   11.588%   $41.5888   $41.59   $41.60   $41.60  
  7 purchase totals = $500.29       $551.3903   $551.40   $551.40   $551.50