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 $44.62   13.421%   $50.6085   $50.61   $50.60   $50.60  
  Purchase #2 $199.38   9.607%   $218.5344   $218.53   $218.55   $218.50  
  Purchase #3 $128.77   3.705%   $133.5409   $133.54   $133.55   $133.50  
  Purchase #4 $130.46   5.800%   $138.0267   $138.03   $138.05   $138.00  
  Purchase #5 $27.67   13.385%   $31.3736   $31.37   $31.35   $31.40  
  Purchase #6 $175.68   4.289%   $183.2149   $183.21   $183.20   $183.20  
  Purchase #7 $40.78   11.714%   $45.557   $45.56   $45.55   $45.60  
  7 purchase totals = $747.36       $800.8560   $800.85   $800.85   $800.80