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 $20.26   4.199%   $21.1107   $21.11   $21.10   $21.10  
  Purchase #2 $162.34   11.414%   $180.8695   $180.87   $180.85   $180.90  
  Purchase #3 $172.31   4.528%   $180.1122   $180.11   $180.10   $180.10  
  Purchase #4 $117.79   3.427%   $121.8267   $121.83   $121.85   $121.80  
  Purchase #5 $136.53   6.869%   $145.9082   $145.91   $145.90   $145.90  
  Purchase #6 $20.91   6.341%   $22.2359   $22.24   $22.25   $22.20  
  Purchase #7 $245.39   13.700%   $279.0084   $279.01   $279.00   $279.00  
  7 purchase totals = $875.53       $951.0716   $951.08   $951.05   $951.00