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 $30.39   3.527%   $31.4619   $31.46   $31.45   $31.50  
  Purchase #2 $75.30   9.691%   $82.5973   $82.60   $82.60   $82.60  
  Purchase #3 $216.68   12.302%   $243.336   $243.34   $243.35   $243.30  
  Purchase #4 $79.74   7.161%   $85.4502   $85.45   $85.45   $85.50  
  Purchase #5 $132.88   12.749%   $149.8209   $149.82   $149.80   $149.80  
  Purchase #6 $224.52   4.686%   $235.041   $235.04   $235.05   $235.00  
  Purchase #7 $86.97   6.619%   $92.7265   $92.73   $92.75   $92.70  
  7 purchase totals = $846.48       $920.4338   $920.44   $920.45   $920.40