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 $138.64   8.435%   $150.3343   $150.33   $150.35   $150.30  
  Purchase #2 $45.31   5.845%   $47.9584   $47.96   $47.95   $48.00  
  Purchase #3 $81.55   4.153%   $84.9368   $84.94   $84.95   $84.90  
  Purchase #4 $85.57   8.337%   $92.704   $92.70   $92.70   $92.70  
  Purchase #5 $139.61   8.773%   $151.858   $151.86   $151.85   $151.90  
  Purchase #6 $122.47   13.442%   $138.9324   $138.93   $138.95   $138.90  
  Purchase #7 $229.94   10.386%   $253.8216   $253.82   $253.80   $253.80  
  7 purchase totals = $843.09       $920.5455   $920.54   $920.55   $920.50