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 $18.84   10.367%   $20.7931   $20.79   $20.80   $20.80  
  Purchase #2 $65.73   10.444%   $72.5948   $72.59   $72.60   $72.60  
  Purchase #3 $192.47   10.173%   $212.05   $212.05   $212.05   $212.10  
  Purchase #4 $243.40   12.238%   $273.1873   $273.19   $273.20   $273.20  
  Purchase #5 $79.25   8.574%   $86.0449   $86.04   $86.05   $86.00  
  Purchase #6 $83.75   11.275%   $93.1928   $93.19   $93.20   $93.20  
  Purchase #7 $141.98   3.555%   $147.0274   $147.03   $147.05   $147.00  
  7 purchase totals = $825.42       $904.8903   $904.88   $904.95   $904.90