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 $47.32   11.701%   $52.8569   $52.86   $52.85   $52.90  
  Purchase #2 $143.67   12.574%   $161.7351   $161.74   $161.75   $161.70  
  Purchase #3 $175.39   5.412%   $184.8821   $184.88   $184.90   $184.90  
  Purchase #4 $12.86   12.532%   $14.4716   $14.47   $14.45   $14.50  
  Purchase #5 $98.81   10.660%   $109.3431   $109.34   $109.35   $109.30  
  Purchase #6 $77.52   8.124%   $83.8177   $83.82   $83.80   $83.80  
  Purchase #7 $212.59   8.608%   $230.8897   $230.89   $230.90   $230.90  
  7 purchase totals = $768.16       $837.9962   $838.00   $838.00   $838.00