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 $20.32   9.468%   $22.2439   $22.24   $22.25   $22.20  
  Purchase #2 $33.99   13.251%   $38.494   $38.49   $38.50   $38.50  
  Purchase #3 $102.76   3.558%   $106.4162   $106.42   $106.40   $106.40  
  Purchase #4 $99.73   10.663%   $110.3642   $110.36   $110.35   $110.40  
  Purchase #5 $21.98   7.664%   $23.6645   $23.66   $23.65   $23.70  
  Purchase #6 $91.79   12.273%   $103.0554   $103.06   $103.05   $103.10  
  Purchase #7 $104.62   10.249%   $115.3425   $115.34   $115.35   $115.30  
  7 purchase totals = $475.19       $519.5807   $519.57   $519.55   $519.60