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 $226.38   10.146%   $249.3485   $249.35   $249.35   $249.30  
  Purchase #2 $183.54   5.369%   $193.3943   $193.39   $193.40   $193.40  
  Purchase #3 $96.76   3.139%   $99.7973   $99.80   $99.80   $99.80  
  Purchase #4 $48.55   5.594%   $51.2659   $51.27   $51.25   $51.30  
  Purchase #5 $189.79   3.144%   $195.757   $195.76   $195.75   $195.80  
  Purchase #6 $124.34   10.871%   $137.857   $137.86   $137.85   $137.90  
  Purchase #7 $38.42   5.754%   $40.6307   $40.63   $40.65   $40.60  
  7 purchase totals = $907.78       $968.0507   $968.06   $968.05   $968.10