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 $24.73   13.175%   $27.9882   $27.99   $28.00   $28.00  
  Purchase #2 $83.59   7.452%   $89.8191   $89.82   $89.80   $89.80  
  Purchase #3 $41.54   10.698%   $45.9839   $45.98   $46.00   $46.00  
  Purchase #4 $23.59   6.352%   $25.0884   $25.09   $25.10   $25.10  
  Purchase #5 $49.75   9.763%   $54.6071   $54.61   $54.60   $54.60  
  Purchase #6 $195.84   7.615%   $210.7532   $210.75   $210.75   $210.80  
  Purchase #7 $227.28   3.179%   $234.5052   $234.51   $234.50   $234.50  
  7 purchase totals = $646.32       $688.7451   $688.75   $688.75   $688.80