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 $75.38   6.393%   $80.199   $80.20   $80.20   $80.20  
  Purchase #2 $103.29   5.217%   $108.6786   $108.68   $108.70   $108.70  
  Purchase #3 $13.94   13.684%   $15.8475   $15.85   $15.85   $15.80  
  Purchase #4 $205.44   10.575%   $227.1653   $227.17   $227.15   $227.20  
  Purchase #5 $16.33   10.219%   $17.9988   $18.00   $18.00   $18.00  
  Purchase #6 $211.39   12.738%   $238.3169   $238.32   $238.30   $238.30  
  Purchase #7 $10.71   6.422%   $11.3978   $11.40   $11.40   $11.40  
  7 purchase totals = $636.48       $699.6039   $699.62   $699.60   $699.60