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 $114.76   8.337%   $124.3275   $124.33   $124.35   $124.30  
  Purchase #2 $37.71   13.145%   $42.667   $42.67   $42.65   $42.70  
  Purchase #3 $110.85   7.820%   $119.5185   $119.52   $119.50   $119.50  
  Purchase #4 $240.61   4.814%   $252.193   $252.19   $252.20   $252.20  
  Purchase #5 $46.94   4.303%   $48.9598   $48.96   $48.95   $49.00  
  Purchase #6 $217.32   9.840%   $238.7043   $238.70   $238.70   $238.70  
  Purchase #7 $246.59   6.709%   $263.1337   $263.13   $263.15   $263.10  
  7 purchase totals = $1,014.78       $1,089.5038   $1,089.50   $1,089.50   $1,089.50