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 $203.46   13.649%   $231.2303   $231.23   $231.25   $231.20  
  Purchase #2 $96.84   8.847%   $105.4074   $105.41   $105.40   $105.40  
  Purchase #3 $149.84   4.474%   $156.5438   $156.54   $156.55   $156.50  
  Purchase #4 $50.53   13.694%   $57.4496   $57.45   $57.45   $57.40  
  Purchase #5 $23.94   5.862%   $25.3434   $25.34   $25.35   $25.30  
  Purchase #6 $61.35   4.613%   $64.1801   $64.18   $64.20   $64.20  
  Purchase #7 $33.66   4.700%   $35.242   $35.24   $35.25   $35.20  
  7 purchase totals = $619.62       $675.3966   $675.39   $675.45   $675.20