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 $123.27   5.294%   $129.7959   $129.80   $129.80   $129.80  
  Purchase #2 $80.47   12.149%   $90.2463   $90.25   $90.25   $90.20  
  Purchase #3 $141.35   7.274%   $151.6318   $151.63   $151.65   $151.60  
  Purchase #4 $17.89   11.466%   $19.9413   $19.94   $19.95   $19.90  
  Purchase #5 $104.93   3.833%   $108.952   $108.95   $108.95   $109.00  
  Purchase #6 $34.43   11.180%   $38.2793   $38.28   $38.30   $38.30  
  Purchase #7 $17.77   8.226%   $19.2318   $19.23   $19.25   $19.20  
  7 purchase totals = $520.11       $558.0784   $558.08   $558.15   $558.00