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 $246.41   11.455%   $274.6363   $274.64   $274.65   $274.60  
  Purchase #2 $58.37   3.597%   $60.4696   $60.47   $60.45   $60.50  
  Purchase #3 $124.81   10.706%   $138.1722   $138.17   $138.15   $138.20  
  Purchase #4 $120.80   5.779%   $127.781   $127.78   $127.80   $127.80  
  Purchase #5 $128.83   11.825%   $144.0641   $144.06   $144.05   $144.10  
  Purchase #6 $45.54   3.862%   $47.2988   $47.30   $47.30   $47.30  
  Purchase #7 $49.68   5.744%   $52.5336   $52.53   $52.55   $52.50  
  7 purchase totals = $774.44       $844.9556   $844.95   $844.95   $845.00