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 $19.99   5.627%   $21.1148   $21.11   $21.10   $21.10  
  Purchase #2 $53.81   3.764%   $55.8354   $55.84   $55.85   $55.80  
  Purchase #3 $6.70   6.440%   $7.1315   $7.13   $7.15   $7.10  
  Purchase #4 $208.37   10.234%   $229.6946   $229.69   $229.70   $229.70  
  Purchase #5 $171.89   10.449%   $189.8508   $189.85   $189.85   $189.90  
  Purchase #6 $157.55   11.299%   $175.3516   $175.35   $175.35   $175.40  
  Purchase #7 $121.69   5.693%   $128.6178   $128.62   $128.60   $128.60  
  7 purchase totals = $740.00       $807.5965   $807.59   $807.60   $807.60