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 $18.44   13.529%   $20.9347   $20.93   $20.95   $20.90  
  Purchase #2 $105.51   11.860%   $118.0235   $118.02   $118.00   $118.00  
  Purchase #3 $63.60   9.182%   $69.4398   $69.44   $69.45   $69.40  
  Purchase #4 $71.88   10.780%   $79.6287   $79.63   $79.65   $79.60  
  Purchase #5 $196.86   6.861%   $210.3666   $210.37   $210.35   $210.40  
  Purchase #6 $48.74   5.738%   $51.5367   $51.54   $51.55   $51.50  
  Purchase #7 $98.92   9.714%   $108.5291   $108.53   $108.55   $108.50  
  7 purchase totals = $603.95       $658.4591   $658.46   $658.50   $658.30