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 $8.53   5.408%   $8.9913   $8.99   $9.00   $9.00  
  Purchase #2 $7.90   7.736%   $8.5111   $8.51   $8.50   $8.50  
  Purchase #3 $22.95   10.808%   $25.4304   $25.43   $25.45   $25.40  
  Purchase #4 $162.57   5.117%   $170.8887   $170.89   $170.90   $170.90  
  Purchase #5 $178.79   12.396%   $200.9528   $200.95   $200.95   $201.00  
  Purchase #6 $219.69   5.534%   $231.8476   $231.85   $231.85   $231.80  
  Purchase #7 $32.87   11.485%   $36.6451   $36.65   $36.65   $36.60  
  7 purchase totals = $633.30       $683.2670   $683.27   $683.30   $683.20