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 $230.35   13.169%   $260.6848   $260.68   $260.70   $260.70  
  Purchase #2 $15.52   8.403%   $16.8241   $16.82   $16.80   $16.80  
  Purchase #3 $137.69   11.608%   $153.6731   $153.67   $153.65   $153.70  
  Purchase #4 $214.95   13.314%   $243.5684   $243.57   $243.55   $243.60  
  Purchase #5 $174.60   6.862%   $186.5811   $186.58   $186.60   $186.60  
  Purchase #6 $72.45   4.174%   $75.4741   $75.47   $75.45   $75.50  
  Purchase #7 $15.78   5.190%   $16.599   $16.60   $16.60   $16.60  
  7 purchase totals = $861.34       $953.4046   $953.39   $953.35   $953.50