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 $19.28   8.693%   $20.956   $20.96   $20.95   $21.00  
  Purchase #2 $12.94   9.784%   $14.206   $14.21   $14.20   $14.20  
  Purchase #3 $39.64   10.790%   $43.9172   $43.92   $43.90   $43.90  
  Purchase #4 $143.79   9.857%   $157.9634   $157.96   $157.95   $158.00  
  Purchase #5 $8.77   5.819%   $9.2803   $9.28   $9.30   $9.30  
  Purchase #6 $180.31   12.561%   $202.9587   $202.96   $202.95   $203.00  
  Purchase #7 $29.68   10.534%   $32.8065   $32.81   $32.80   $32.80  
  7 purchase totals = $434.41       $482.0881   $482.10   $482.05   $482.20