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 $62.97   3.713%   $65.3081   $65.31   $65.30   $65.30  
  Purchase #2 $54.78   12.664%   $61.7173   $61.72   $61.70   $61.70  
  Purchase #3 $79.85   10.174%   $87.9739   $87.97   $87.95   $88.00  
  Purchase #4 $233.38   7.270%   $250.3467   $250.35   $250.35   $250.30  
  Purchase #5 $132.36   4.465%   $138.2699   $138.27   $138.25   $138.30  
  Purchase #6 $242.95   4.487%   $253.8512   $253.85   $253.85   $253.90  
  Purchase #7 $117.27   8.166%   $126.8463   $126.85   $126.85   $126.80  
  7 purchase totals = $923.56       $984.3134   $984.32   $984.25   $984.30