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 $225.56   12.818%   $254.4723   $254.47   $254.45   $254.50  
  Purchase #2 $143.81   11.247%   $159.9843   $159.98   $160.00   $160.00  
  Purchase #3 $125.27   6.578%   $133.5103   $133.51   $133.50   $133.50  
  Purchase #4 $184.64   12.661%   $208.0173   $208.02   $208.00   $208.00  
  Purchase #5 $238.71   5.620%   $252.1255   $252.13   $252.15   $252.10  
  Purchase #6 $96.65   7.728%   $104.1191   $104.12   $104.10   $104.10  
  Purchase #7 $119.49   5.113%   $125.5995   $125.60   $125.60   $125.60  
  7 purchase totals = $1,134.13       $1,237.8283   $1,237.83   $1,237.80   $1,237.80