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 $206.94   13.378%   $234.6244   $234.62   $234.60   $234.60  
  Purchase #2 $53.91   9.169%   $58.853   $58.85   $58.85   $58.90  
  Purchase #3 $181.66   10.773%   $201.2302   $201.23   $201.25   $201.20  
  Purchase #4 $243.72   4.226%   $254.0196   $254.02   $254.00   $254.00  
  Purchase #5 $56.99   8.247%   $61.69   $61.69   $61.70   $61.70  
  Purchase #6 $103.29   11.483%   $115.1508   $115.15   $115.15   $115.20  
  Purchase #7 $47.65   9.373%   $52.1162   $52.12   $52.10   $52.10  
  7 purchase totals = $894.16       $977.6842   $977.68   $977.65   $977.70