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 $246.34   12.446%   $276.9995   $277.00   $277.00   $277.00  
  Purchase #2 $174.77   8.810%   $190.1672   $190.17   $190.15   $190.20  
  Purchase #3 $20.84   9.299%   $22.7779   $22.78   $22.80   $22.80  
  Purchase #4 $202.58   5.227%   $213.1689   $213.17   $213.15   $213.20  
  Purchase #5 $52.61   13.105%   $59.5045   $59.50   $59.50   $59.50  
  Purchase #6 $11.48   13.633%   $13.0451   $13.05   $13.05   $13.00  
  Purchase #7 $74.36   6.631%   $79.2908   $79.29   $79.30   $79.30  
  7 purchase totals = $782.98       $854.9539   $854.96   $854.95   $855.00