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 $236.66   11.821%   $264.6356   $264.64   $264.65   $264.60  
  Purchase #2 $127.97   7.178%   $137.1557   $137.16   $137.15   $137.20  
  Purchase #3 $119.94   6.773%   $128.0635   $128.06   $128.05   $128.10  
  Purchase #4 $195.65   4.508%   $204.4699   $204.47   $204.45   $204.50  
  Purchase #5 $127.28   6.483%   $135.5316   $135.53   $135.55   $135.50  
  Purchase #6 $32.61   12.434%   $36.6647   $36.66   $36.65   $36.70  
  Purchase #7 $98.63   13.253%   $111.7014   $111.70   $111.70   $111.70  
  7 purchase totals = $938.74       $1,018.2224   $1,018.22   $1,018.20   $1,018.30