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 $17.73   12.574%   $19.9594   $19.96   $19.95   $20.00  
  Purchase #2 $129.50   5.651%   $136.818   $136.82   $136.80   $136.80  
  Purchase #3 $202.56   12.729%   $228.3439   $228.34   $228.35   $228.30  
  Purchase #4 $196.89   13.634%   $223.734   $223.73   $223.75   $223.70  
  Purchase #5 $6.92   6.665%   $7.3812   $7.38   $7.40   $7.40  
  Purchase #6 $28.27   3.444%   $29.2436   $29.24   $29.25   $29.20  
  Purchase #7 $219.61   11.697%   $245.2978   $245.30   $245.30   $245.30  
  7 purchase totals = $801.48       $890.7779   $890.77   $890.80   $890.70