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 $27.47   12.540%   $30.9147   $30.91   $30.90   $30.90  
  Purchase #2 $174.34   8.681%   $189.4745   $189.47   $189.45   $189.50  
  Purchase #3 $95.31   6.533%   $101.5366   $101.54   $101.55   $101.50  
  Purchase #4 $108.60   8.396%   $117.7181   $117.72   $117.70   $117.70  
  Purchase #5 $147.66   5.484%   $155.7577   $155.76   $155.75   $155.80  
  Purchase #6 $79.36   5.148%   $83.4455   $83.45   $83.45   $83.40  
  Purchase #7 $175.79   10.566%   $194.364   $194.36   $194.35   $194.40  
  7 purchase totals = $808.53       $873.2111   $873.21   $873.15   $873.20