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 $8.80   12.345%   $9.8864   $9.89   $9.90   $9.90  
  Purchase #2 $151.89   12.298%   $170.5694   $170.57   $170.55   $170.60  
  Purchase #3 $215.36   13.135%   $243.6475   $243.65   $243.65   $243.60  
  Purchase #4 $55.32   11.212%   $61.5225   $61.52   $61.50   $61.50  
  Purchase #5 $63.49   6.670%   $67.7248   $67.72   $67.70   $67.70  
  Purchase #6 $42.44   11.850%   $47.4691   $47.47   $47.45   $47.50  
  Purchase #7 $94.70   4.519%   $98.9795   $98.98   $99.00   $99.00  
  7 purchase totals = $632.00       $699.7992   $699.80   $699.75   $699.80