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 $120.52   3.519%   $124.7611   $124.76   $124.75   $124.80  
  Purchase #2 $87.71   5.598%   $92.62   $92.62   $92.60   $92.60  
  Purchase #3 $154.48   8.579%   $167.7328   $167.73   $167.75   $167.70  
  Purchase #4 $230.77   11.728%   $257.8347   $257.83   $257.85   $257.80  
  Purchase #5 $125.93   9.814%   $138.2888   $138.29   $138.30   $138.30  
  Purchase #6 $11.48   9.795%   $12.6045   $12.60   $12.60   $12.60  
  Purchase #7 $118.31   4.514%   $123.6505   $123.65   $123.65   $123.70  
  7 purchase totals = $849.20       $917.4924   $917.48   $917.50   $917.50