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 $61.25   8.435%   $66.4164   $66.42   $66.40   $66.40  
  Purchase #2 $210.45   6.182%   $223.46   $223.46   $223.45   $223.50  
  Purchase #3 $127.38   5.855%   $134.8381   $134.84   $134.85   $134.80  
  Purchase #4 $8.32   11.818%   $9.3033   $9.30   $9.30   $9.30  
  Purchase #5 $23.40   13.430%   $26.5426   $26.54   $26.55   $26.50  
  Purchase #6 $106.67   11.612%   $119.0565   $119.06   $119.05   $119.10  
  Purchase #7 $97.59   6.372%   $103.8084   $103.81   $103.80   $103.80  
  7 purchase totals = $635.06       $683.4253   $683.43   $683.40   $683.40