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 $75.50   7.133%   $80.8854   $80.89   $80.90   $80.90  
  Purchase #2 $21.37   4.776%   $22.3906   $22.39   $22.40   $22.40  
  Purchase #3 $127.28   6.231%   $135.2108   $135.21   $135.20   $135.20  
  Purchase #4 $244.45   13.873%   $278.3625   $278.36   $278.35   $278.40  
  Purchase #5 $64.70   6.324%   $68.7916   $68.79   $68.80   $68.80  
  Purchase #6 $116.78   9.226%   $127.5541   $127.55   $127.55   $127.60  
  Purchase #7 $79.42   11.486%   $88.5422   $88.54   $88.55   $88.50  
  7 purchase totals = $729.50       $801.7372   $801.73   $801.75   $801.80