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 $171.62   3.326%   $177.3281   $177.33   $177.35   $177.30  
  Purchase #2 $81.56   9.394%   $89.2217   $89.22   $89.20   $89.20  
  Purchase #3 $223.79   12.133%   $250.9424   $250.94   $250.95   $250.90  
  Purchase #4 $73.71   8.701%   $80.1235   $80.12   $80.10   $80.10  
  Purchase #5 $70.57   11.225%   $78.4915   $78.49   $78.50   $78.50  
  Purchase #6 $227.67   3.303%   $235.1899   $235.19   $235.20   $235.20  
  Purchase #7 $74.48   7.655%   $80.1814   $80.18   $80.20   $80.20  
  7 purchase totals = $923.40       $991.4785   $991.47   $991.50   $991.40