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 $9.35   6.612%   $9.9682   $9.97   $9.95   $10.00  
  Purchase #2 $167.92   13.611%   $190.7756   $190.78   $190.80   $190.80  
  Purchase #3 $64.35   12.768%   $72.5662   $72.57   $72.55   $72.60  
  Purchase #4 $175.90   7.566%   $189.2086   $189.21   $189.20   $189.20  
  Purchase #5 $105.65   3.111%   $108.9368   $108.94   $108.95   $108.90  
  Purchase #6 $53.44   4.668%   $55.9346   $55.93   $55.95   $55.90  
  Purchase #7 $18.79   9.164%   $20.5119   $20.51   $20.50   $20.50  
  7 purchase totals = $595.40       $647.9019   $647.91   $647.90   $647.90