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 $115.31   5.667%   $121.8446   $121.84   $121.85   $121.80  
  Purchase #2 $100.63   8.315%   $108.9974   $109.00   $109.00   $109.00  
  Purchase #3 $63.41   10.778%   $70.2443   $70.24   $70.25   $70.20  
  Purchase #4 $188.37   7.829%   $203.1175   $203.12   $203.10   $203.10  
  Purchase #5 $238.67   7.406%   $256.3459   $256.35   $256.35   $256.30  
  Purchase #6 $144.58   5.670%   $152.7777   $152.78   $152.80   $152.80  
  Purchase #7 $77.78   10.169%   $85.6894   $85.69   $85.70   $85.70  
  7 purchase totals = $928.75       $999.0168   $999.02   $999.05   $998.90