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 $124.97   10.658%   $138.2893   $138.29   $138.30   $138.30  
  Purchase #2 $89.26   7.485%   $95.9411   $95.94   $95.95   $95.90  
  Purchase #3 $73.42   5.489%   $77.45   $77.45   $77.45   $77.50  
  Purchase #4 $64.60   8.197%   $69.8953   $69.90   $69.90   $69.90  
  Purchase #5 $191.34   11.359%   $213.0743   $213.07   $213.05   $213.10  
  Purchase #6 $155.93   4.461%   $162.886   $162.89   $162.90   $162.90  
  Purchase #7 $210.29   4.149%   $219.0149   $219.01   $219.00   $219.00  
  7 purchase totals = $909.81       $976.5509   $976.55   $976.55   $976.60