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 $61.44   12.127%   $68.8908   $68.89   $68.90   $68.90  
  Purchase #2 $78.29   12.838%   $88.3409   $88.34   $88.35   $88.30  
  Purchase #3 $244.86   7.722%   $263.7681   $263.77   $263.75   $263.80  
  Purchase #4 $211.71   13.681%   $240.674   $240.67   $240.65   $240.70  
  Purchase #5 $213.52   12.848%   $240.953   $240.95   $240.95   $241.00  
  Purchase #6 $11.45   8.257%   $12.3954   $12.40   $12.40   $12.40  
  Purchase #7 $7.58   6.206%   $8.0504   $8.05   $8.05   $8.10  
  7 purchase totals = $828.85       $923.0726   $923.07   $923.05   $923.20