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 $171.94   9.404%   $188.1092   $188.11   $188.10   $188.10  
  Purchase #2 $140.33   7.673%   $151.0975   $151.10   $151.10   $151.10  
  Purchase #3 $80.38   13.760%   $91.4403   $91.44   $91.45   $91.40  
  Purchase #4 $201.33   7.596%   $216.623   $216.62   $216.60   $216.60  
  Purchase #5 $21.53   7.843%   $23.2186   $23.22   $23.20   $23.20  
  Purchase #6 $131.79   7.560%   $141.7533   $141.75   $141.75   $141.80  
  Purchase #7 $71.60   8.447%   $77.6481   $77.65   $77.65   $77.60  
  7 purchase totals = $818.90       $889.8900   $889.89   $889.85   $889.80