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 $19.62   8.237%   $21.2361   $21.24   $21.25   $21.20  
  Purchase #2 $9.90   6.745%   $10.5678   $10.57   $10.55   $10.60  
  Purchase #3 $19.53   6.493%   $20.7981   $20.80   $20.80   $20.80  
  Purchase #4 $35.73   9.658%   $39.1808   $39.18   $39.20   $39.20  
  Purchase #5 $100.34   7.571%   $107.9367   $107.94   $107.95   $107.90  
  Purchase #6 $209.84   10.456%   $231.7809   $231.78   $231.80   $231.80  
  Purchase #7 $212.70   12.160%   $238.5643   $238.56   $238.55   $238.60  
  7 purchase totals = $607.66       $670.0647   $670.07   $670.10   $670.10