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 $149.74   3.666%   $155.2295   $155.23   $155.25   $155.20  
  Purchase #2 $170.96   12.331%   $192.0411   $192.04   $192.05   $192.00  
  Purchase #3 $83.84   7.871%   $90.439   $90.44   $90.45   $90.40  
  Purchase #4 $22.61   7.543%   $24.3155   $24.32   $24.30   $24.30  
  Purchase #5 $192.76   10.742%   $213.4663   $213.47   $213.45   $213.50  
  Purchase #6 $174.70   6.268%   $185.6502   $185.65   $185.65   $185.70  
  Purchase #7 $25.74   9.748%   $28.2491   $28.25   $28.25   $28.20  
  7 purchase totals = $820.35       $889.3907   $889.40   $889.40   $889.30