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 $85.45   5.317%   $89.9934   $89.99   $90.00   $90.00  
  Purchase #2 $125.31   10.874%   $138.9362   $138.94   $138.95   $138.90  
  Purchase #3 $111.65   5.581%   $117.8812   $117.88   $117.90   $117.90  
  Purchase #4 $125.43   3.216%   $129.4638   $129.46   $129.45   $129.50  
  Purchase #5 $80.29   12.527%   $90.3479   $90.35   $90.35   $90.30  
  Purchase #6 $74.56   13.508%   $84.6316   $84.63   $84.65   $84.60  
  Purchase #7 $59.79   4.728%   $62.6169   $62.62   $62.60   $62.60  
  7 purchase totals = $662.48       $713.8710   $713.87   $713.90   $713.80