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 $20.79   5.329%   $21.8979   $21.90   $21.90   $21.90  
  Purchase #2 $100.56   5.734%   $106.3261   $106.33   $106.35   $106.30  
  Purchase #3 $156.29   8.150%   $169.0276   $169.03   $169.05   $169.00  
  Purchase #4 $137.45   8.739%   $149.4618   $149.46   $149.45   $149.50  
  Purchase #5 $232.75   11.231%   $258.8902   $258.89   $258.90   $258.90  
  Purchase #6 $62.67   12.481%   $70.4918   $70.49   $70.50   $70.50  
  Purchase #7 $5.93   12.281%   $6.6583   $6.66   $6.65   $6.70  
  7 purchase totals = $716.44       $782.7537   $782.76   $782.80   $782.80