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 $226.55   7.241%   $242.9545   $242.95   $242.95   $243.00  
  Purchase #2 $88.62   3.595%   $91.8059   $91.81   $91.80   $91.80  
  Purchase #3 $72.44   3.433%   $74.9269   $74.93   $74.95   $74.90  
  Purchase #4 $127.86   5.494%   $134.8846   $134.88   $134.90   $134.90  
  Purchase #5 $57.46   7.185%   $61.5885   $61.59   $61.60   $61.60  
  Purchase #6 $86.46   13.705%   $98.3093   $98.31   $98.30   $98.30  
  Purchase #7 $165.88   12.814%   $187.1359   $187.14   $187.15   $187.10  
  7 purchase totals = $825.27       $891.6056   $891.61   $891.65   $891.60