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 $173.99   8.276%   $188.3894   $188.39   $188.40   $188.40  
  Purchase #2 $75.56   12.256%   $84.8206   $84.82   $84.80   $84.80  
  Purchase #3 $7.39   6.717%   $7.8864   $7.89   $7.90   $7.90  
  Purchase #4 $132.62   4.177%   $138.1595   $138.16   $138.15   $138.20  
  Purchase #5 $86.78   5.351%   $91.4236   $91.42   $91.40   $91.40  
  Purchase #6 $144.87   5.224%   $152.438   $152.44   $152.45   $152.40  
  Purchase #7 $134.54   10.302%   $148.4003   $148.40   $148.40   $148.40  
  7 purchase totals = $755.75       $811.5178   $811.52   $811.50   $811.50