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 $188.53   12.147%   $211.4307   $211.43   $211.45   $211.40  
  Purchase #2 $99.48   11.228%   $110.6496   $110.65   $110.65   $110.60  
  Purchase #3 $63.71   11.368%   $70.9526   $70.95   $70.95   $71.00  
  Purchase #4 $61.82   11.549%   $68.9596   $68.96   $68.95   $69.00  
  Purchase #5 $57.71   5.705%   $61.0024   $61.00   $61.00   $61.00  
  Purchase #6 $111.43   8.544%   $120.9506   $120.95   $120.95   $121.00  
  Purchase #7 $205.47   7.490%   $220.8597   $220.86   $220.85   $220.90  
  7 purchase totals = $788.15       $864.8052   $864.80   $864.80   $864.90