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 $96.99   13.164%   $109.7578   $109.76   $109.75   $109.80  
  Purchase #2 $21.47   13.517%   $24.3721   $24.37   $24.35   $24.40  
  Purchase #3 $13.60   11.850%   $15.2116   $15.21   $15.20   $15.20  
  Purchase #4 $186.98   12.528%   $210.4049   $210.40   $210.40   $210.40  
  Purchase #5 $115.56   7.824%   $124.6014   $124.60   $124.60   $124.60  
  Purchase #6 $74.68   8.401%   $80.9539   $80.95   $80.95   $81.00  
  Purchase #7 $18.86   4.666%   $19.74   $19.74   $19.75   $19.70  
  7 purchase totals = $528.14       $585.0417   $585.03   $585.00   $585.10