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 $226.29   13.825%   $257.5746   $257.57   $257.55   $257.60  
  Purchase #2 $140.92   13.665%   $160.1767   $160.18   $160.20   $160.20  
  Purchase #3 $146.29   6.833%   $156.286   $156.29   $156.30   $156.30  
  Purchase #4 $112.67   4.622%   $117.8776   $117.88   $117.90   $117.90  
  Purchase #5 $100.49   9.847%   $110.3853   $110.39   $110.40   $110.40  
  Purchase #6 $146.43   12.324%   $164.476   $164.48   $164.50   $164.50  
  Purchase #7 $7.93   5.842%   $8.3933   $8.39   $8.40   $8.40  
  7 purchase totals = $881.02       $975.1695   $975.18   $975.25   $975.30