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 $21.28   4.540%   $22.2461   $22.25   $22.25   $22.20  
  Purchase #2 $19.85   7.733%   $21.385   $21.39   $21.40   $21.40  
  Purchase #3 $210.55   4.690%   $220.4248   $220.42   $220.40   $220.40  
  Purchase #4 $80.74   5.253%   $84.9813   $84.98   $85.00   $85.00  
  Purchase #5 $128.53   7.276%   $137.8818   $137.88   $137.90   $137.90  
  Purchase #6 $144.68   13.609%   $164.3695   $164.37   $164.35   $164.40  
  Purchase #7 $169.41   8.758%   $184.2469   $184.25   $184.25   $184.20  
  7 purchase totals = $775.04       $835.5354   $835.54   $835.55   $835.50