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 $211.76   12.854%   $238.9796   $238.98   $239.00   $239.00  
  Purchase #2 $51.94   5.587%   $54.8419   $54.84   $54.85   $54.80  
  Purchase #3 $174.82   4.424%   $182.554   $182.55   $182.55   $182.60  
  Purchase #4 $88.33   10.670%   $97.7548   $97.75   $97.75   $97.80  
  Purchase #5 $216.47   10.313%   $238.7946   $238.79   $238.80   $238.80  
  Purchase #6 $78.60   7.587%   $84.5634   $84.56   $84.55   $84.60  
  Purchase #7 $56.33   9.157%   $61.4881   $61.49   $61.50   $61.50  
  7 purchase totals = $878.25       $958.9764   $958.96   $959.00   $959.10