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 $83.78   7.400%   $89.9797   $89.98   $90.00   $90.00  
  Purchase #2 $22.76   3.118%   $23.4697   $23.47   $23.45   $23.50  
  Purchase #3 $123.32   4.170%   $128.4624   $128.46   $128.45   $128.50  
  Purchase #4 $112.77   7.617%   $121.3597   $121.36   $121.35   $121.40  
  Purchase #5 $174.61   7.421%   $187.5678   $187.57   $187.55   $187.60  
  Purchase #6 $11.53   9.140%   $12.5838   $12.58   $12.60   $12.60  
  Purchase #7 $130.36   8.206%   $141.0573   $141.06   $141.05   $141.10  
  7 purchase totals = $659.13       $704.4804   $704.48   $704.45   $704.70