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 $118.73   3.774%   $123.2109   $123.21   $123.20   $123.20  
  Purchase #2 $149.71   8.814%   $162.9054   $162.91   $162.90   $162.90  
  Purchase #3 $24.68   13.572%   $28.0296   $28.03   $28.05   $28.00  
  Purchase #4 $101.87   8.541%   $110.5707   $110.57   $110.55   $110.60  
  Purchase #5 $180.66   13.149%   $204.415   $204.42   $204.40   $204.40  
  Purchase #6 $193.84   13.329%   $219.6769   $219.68   $219.70   $219.70  
  Purchase #7 $127.35   11.852%   $142.4435   $142.44   $142.45   $142.40  
  7 purchase totals = $896.84       $991.2520   $991.26   $991.25   $991.20