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 $58.85   12.119%   $65.982   $65.98   $66.00   $66.00  
  Purchase #2 $181.44   6.219%   $192.7238   $192.72   $192.70   $192.70  
  Purchase #3 $154.54   10.453%   $170.6941   $170.69   $170.70   $170.70  
  Purchase #4 $9.90   8.149%   $10.7068   $10.71   $10.70   $10.70  
  Purchase #5 $92.60   12.174%   $103.8731   $103.87   $103.85   $103.90  
  Purchase #6 $226.44   8.173%   $244.9469   $244.95   $244.95   $244.90  
  Purchase #7 $230.34   8.194%   $249.2141   $249.21   $249.20   $249.20  
  7 purchase totals = $954.11       $1,038.1408   $1,038.13   $1,038.10   $1,038.10