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 $65.49   12.318%   $73.5571   $73.56   $73.55   $73.60  
  Purchase #2 $93.69   6.441%   $99.7246   $99.72   $99.70   $99.70  
  Purchase #3 $122.71   13.242%   $138.9593   $138.96   $138.95   $139.00  
  Purchase #4 $238.86   3.333%   $246.8212   $246.82   $246.80   $246.80  
  Purchase #5 $101.83   4.687%   $106.6028   $106.60   $106.60   $106.60  
  Purchase #6 $48.82   6.825%   $52.152   $52.15   $52.15   $52.20  
  Purchase #7 $79.57   10.853%   $88.2057   $88.21   $88.20   $88.20  
  7 purchase totals = $750.97       $806.0227   $806.02   $805.95   $806.10