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 $165.93   12.277%   $186.3012   $186.30   $186.30   $186.30  
  Purchase #2 $63.76   8.690%   $69.3007   $69.30   $69.30   $69.30  
  Purchase #3 $65.90   10.134%   $72.5783   $72.58   $72.60   $72.60  
  Purchase #4 $202.34   12.276%   $227.1793   $227.18   $227.20   $227.20  
  Purchase #5 $129.50   12.634%   $145.861   $145.86   $145.85   $145.90  
  Purchase #6 $9.74   11.426%   $10.8529   $10.85   $10.85   $10.90  
  Purchase #7 $97.89   10.529%   $108.1968   $108.20   $108.20   $108.20  
  7 purchase totals = $735.06       $820.2702   $820.27   $820.30   $820.40