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 $20.29   7.596%   $21.8312   $21.83   $21.85   $21.80  
  Purchase #2 $31.42   7.552%   $33.7928   $33.79   $33.80   $33.80  
  Purchase #3 $23.51   3.689%   $24.3773   $24.38   $24.40   $24.40  
  Purchase #4 $170.75   8.789%   $185.7572   $185.76   $185.75   $185.80  
  Purchase #5 $12.74   8.503%   $13.8233   $13.82   $13.80   $13.80  
  Purchase #6 $10.39   11.450%   $11.5797   $11.58   $11.60   $11.60  
  Purchase #7 $52.80   7.373%   $56.6929   $56.69   $56.70   $56.70  
  7 purchase totals = $321.90       $347.8544   $347.85   $347.90   $347.90