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 $87.81   6.838%   $93.8144   $93.81   $93.80   $93.80  
  Purchase #2 $44.60   13.864%   $50.7833   $50.78   $50.80   $50.80  
  Purchase #3 $135.41   3.378%   $139.9841   $139.98   $140.00   $140.00  
  Purchase #4 $61.40   11.677%   $68.5697   $68.57   $68.55   $68.60  
  Purchase #5 $29.55   7.599%   $31.7955   $31.80   $31.80   $31.80  
  Purchase #6 $54.40   6.576%   $57.9773   $57.98   $58.00   $58.00  
  Purchase #7 $97.36   11.151%   $108.2166   $108.22   $108.20   $108.20  
  7 purchase totals = $510.53       $551.1409   $551.14   $551.15   $551.20