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 $80.69   3.797%   $83.7538   $83.75   $83.75   $83.80  
  Purchase #2 $23.69   12.340%   $26.6133   $26.61   $26.60   $26.60  
  Purchase #3 $86.76   4.823%   $90.9444   $90.94   $90.95   $90.90  
  Purchase #4 $194.92   7.142%   $208.8412   $208.84   $208.85   $208.80  
  Purchase #5 $190.86   6.367%   $203.0121   $203.01   $203.00   $203.00  
  Purchase #6 $233.27   4.243%   $243.1676   $243.17   $243.15   $243.20  
  Purchase #7 $126.59   11.349%   $140.9567   $140.96   $140.95   $141.00  
  7 purchase totals = $936.78       $997.2891   $997.28   $997.25   $997.30