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 $36.73   6.572%   $39.1439   $39.14   $39.15   $39.10  
  Purchase #2 $89.37   7.550%   $96.1174   $96.12   $96.10   $96.10  
  Purchase #3 $144.33   12.553%   $162.4477   $162.45   $162.45   $162.40  
  Purchase #4 $45.76   5.827%   $48.4264   $48.43   $48.45   $48.40  
  Purchase #5 $140.31   11.821%   $156.896   $156.90   $156.90   $156.90  
  Purchase #6 $78.62   11.141%   $87.3791   $87.38   $87.40   $87.40  
  Purchase #7 $8.78   11.715%   $9.8086   $9.81   $9.80   $9.80  
  7 purchase totals = $543.90       $600.2191   $600.23   $600.25   $600.10