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 $15.94   13.690%   $18.1222   $18.12   $18.10   $18.10  
  Purchase #2 $41.79   8.753%   $45.4479   $45.45   $45.45   $45.40  
  Purchase #3 $91.61   9.179%   $100.0189   $100.02   $100.00   $100.00  
  Purchase #4 $171.96   6.784%   $183.6258   $183.63   $183.65   $183.60  
  Purchase #5 $7.83   4.251%   $8.1629   $8.16   $8.15   $8.20  
  Purchase #6 $149.90   13.586%   $170.2654   $170.27   $170.25   $170.30  
  Purchase #7 $137.85   13.717%   $156.7589   $156.76   $156.75   $156.80  
  7 purchase totals = $616.88       $682.4020   $682.41   $682.35   $682.40