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 $58.52   4.592%   $61.2072   $61.21   $61.20   $61.20  
  Purchase #2 $26.94   11.767%   $30.11   $30.11   $30.10   $30.10  
  Purchase #3 $86.92   7.819%   $93.7163   $93.72   $93.70   $93.70  
  Purchase #4 $25.97   4.850%   $27.2295   $27.23   $27.25   $27.20  
  Purchase #5 $194.39   9.219%   $212.3108   $212.31   $212.30   $212.30  
  Purchase #6 $103.67   5.704%   $109.5833   $109.58   $109.60   $109.60  
  Purchase #7 $16.32   7.617%   $17.5631   $17.56   $17.55   $17.60  
  7 purchase totals = $512.73       $551.7202   $551.72   $551.70   $551.70