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 $35.46   7.779%   $38.2184   $38.22   $38.20   $38.20  
  Purchase #2 $25.43   9.604%   $27.8723   $27.87   $27.85   $27.90  
  Purchase #3 $152.75   4.609%   $159.7902   $159.79   $159.80   $159.80  
  Purchase #4 $108.74   3.402%   $112.4393   $112.44   $112.45   $112.40  
  Purchase #5 $128.37   11.624%   $143.2917   $143.29   $143.30   $143.30  
  Purchase #6 $80.57   11.845%   $90.1135   $90.11   $90.10   $90.10  
  Purchase #7 $65.88   4.519%   $68.8571   $68.86   $68.85   $68.90  
  7 purchase totals = $597.20       $640.5825   $640.58   $640.55   $640.60