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 $125.54   10.594%   $138.8397   $138.84   $138.85   $138.80  
  Purchase #2 $153.93   5.869%   $162.9642   $162.96   $162.95   $163.00  
  Purchase #3 $156.49   4.822%   $164.0359   $164.04   $164.05   $164.00  
  Purchase #4 $149.48   13.400%   $169.5103   $169.51   $169.50   $169.50  
  Purchase #5 $44.89   9.434%   $49.1249   $49.12   $49.10   $49.10  
  Purchase #6 $120.96   5.343%   $127.4229   $127.42   $127.40   $127.40  
  Purchase #7 $142.45   9.692%   $156.2563   $156.26   $156.25   $156.30  
  7 purchase totals = $893.74       $968.1542   $968.15   $968.10   $968.10