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 $130.63   6.304%   $138.8649   $138.86   $138.85   $138.90  
  Purchase #2 $148.33   13.812%   $168.8173   $168.82   $168.80   $168.80  
  Purchase #3 $7.68   4.860%   $8.0532   $8.05   $8.05   $8.10  
  Purchase #4 $163.77   13.598%   $186.0394   $186.04   $186.05   $186.00  
  Purchase #5 $115.92   13.650%   $131.7431   $131.74   $131.75   $131.70  
  Purchase #6 $31.75   7.117%   $34.0096   $34.01   $34.00   $34.00  
  Purchase #7 $235.32   4.409%   $245.6953   $245.70   $245.70   $245.70  
  7 purchase totals = $833.40       $913.2228   $913.22   $913.20   $913.20