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 $20.27   5.342%   $21.3528   $21.35   $21.35   $21.40  
  Purchase #2 $10.69   9.329%   $11.6873   $11.69   $11.70   $11.70  
  Purchase #3 $22.39   10.154%   $24.6635   $24.66   $24.65   $24.70  
  Purchase #4 $182.67   5.779%   $193.2265   $193.23   $193.25   $193.20  
  Purchase #5 $209.39   4.854%   $219.5538   $219.55   $219.55   $219.60  
  Purchase #6 $153.80   11.176%   $170.9887   $170.99   $171.00   $171.00  
  Purchase #7 $43.26   12.853%   $48.8202   $48.82   $48.80   $48.80  
  7 purchase totals = $642.47       $690.2928   $690.29   $690.30   $690.40