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 $205.42   10.194%   $226.3605   $226.36   $226.35   $226.40  
  Purchase #2 $80.78   5.145%   $84.9361   $84.94   $84.95   $84.90  
  Purchase #3 $37.94   3.866%   $39.4068   $39.41   $39.40   $39.40  
  Purchase #4 $60.55   7.754%   $65.245   $65.25   $65.25   $65.20  
  Purchase #5 $95.35   9.155%   $104.0793   $104.08   $104.10   $104.10  
  Purchase #6 $46.33   12.225%   $51.9938   $51.99   $52.00   $52.00  
  Purchase #7 $35.82   13.765%   $40.7506   $40.75   $40.75   $40.80  
  7 purchase totals = $562.19       $612.7721   $612.78   $612.80   $612.80