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 $19.93   7.134%   $21.3518   $21.35   $21.35   $21.40  
  Purchase #2 $11.91   10.657%   $13.1792   $13.18   $13.20   $13.20  
  Purchase #3 $222.51   13.552%   $252.6646   $252.66   $252.65   $252.70  
  Purchase #4 $7.82   5.694%   $8.2653   $8.27   $8.25   $8.30  
  Purchase #5 $231.86   7.603%   $249.4883   $249.49   $249.50   $249.50  
  Purchase #6 $234.97   10.755%   $260.241   $260.24   $260.25   $260.20  
  Purchase #7 $247.31   12.382%   $277.9319   $277.93   $277.95   $277.90  
  7 purchase totals = $976.31       $1,083.1221   $1,083.12   $1,083.15   $1,083.20