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 $16.91   6.421%   $17.9958   $18.00   $18.00   $18.00  
  Purchase #2 $77.46   8.494%   $84.0395   $84.04   $84.05   $84.00  
  Purchase #3 $171.66   6.666%   $183.1029   $183.10   $183.10   $183.10  
  Purchase #4 $165.60   11.614%   $184.8328   $184.83   $184.85   $184.80  
  Purchase #5 $95.62   8.753%   $103.9896   $103.99   $104.00   $104.00  
  Purchase #6 $79.96   11.348%   $89.0339   $89.03   $89.05   $89.00  
  Purchase #7 $12.62   9.391%   $13.8051   $13.81   $13.80   $13.80  
  7 purchase totals = $619.83       $676.7996   $676.80   $676.85   $676.70