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 $99.75   11.396%   $111.1175   $111.12   $111.10   $111.10  
  Purchase #2 $5.80   10.850%   $6.4293   $6.43   $6.45   $6.40  
  Purchase #3 $21.79   4.184%   $22.7017   $22.70   $22.70   $22.70  
  Purchase #4 $140.46   4.716%   $147.0841   $147.08   $147.10   $147.10  
  Purchase #5 $98.29   12.189%   $110.2706   $110.27   $110.25   $110.30  
  Purchase #6 $173.79   13.578%   $197.3872   $197.39   $197.40   $197.40  
  Purchase #7 $136.77   5.223%   $143.9135   $143.91   $143.90   $143.90  
  7 purchase totals = $676.65       $738.9039   $738.90   $738.90   $738.90